chore: correct the MSRV check (#934)

## Motivation

The current MSRV check is lazybones, it's meaningless unless we override
toolchain. Actually, we cannot compile tracing crates with 1.39.0, it
should work with 1.44.0 or later because of the `protobuf` crate and
others. 
 
## Solution

This enforces to override toolchain and updates MSRV to 1.40.0, this
should compile all the crates fine. And replaces
`actions/checkout@master` with `main` as they've renamed the default
branch name to `main`.

.github/workflows/CI.yml

@@ -12,13 +12,14 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        rust: [stable, 1.39.0]
+        rust: [stable, 1.40.0]
     steps:
-    - uses: actions/checkout@master
+    - uses: actions/checkout@main
     - uses: actions-rs/toolchain@v1
       with:
         toolchain: ${{ matrix.rust }}
         profile: minimal
+        override: true
     - name: Check
       uses: actions-rs/cargo@v1
       with:
@@ -66,11 +67,12 @@ jobs:
         #- tracing
         #- tracing-subscriber
     steps:
-    - uses: actions/checkout@master
+    - uses: actions/checkout@main
     - uses: actions-rs/toolchain@v1
       with:
         toolchain: stable
         profile: minimal
+        override: true
     - name: Fetch latest release version of cargo-hack
       run: |
         mkdir -p .github/caching
@@ -102,11 +104,12 @@ jobs:
         - std log-always
         - std
     steps:
-    - uses: actions/checkout@master
+    - uses: actions/checkout@main
     - uses: actions-rs/toolchain@v1
       with:
         toolchain: stable
         profile: minimal
+        override: true
     - name: cargo check
       working-directory: tracing
       run: cargo check --no-default-features --features "${{ matrix.featureset }}"
@@ -126,11 +129,12 @@ jobs:
         - registry
         - env-filter
     steps:
-    - uses: actions/checkout@master
+    - uses: actions/checkout@main
     - uses: actions-rs/toolchain@v1
       with:
         toolchain: stable
         profile: minimal
+        override: true
     - name: cargo check
       working-directory: tracing-subscriber
       run: cargo check --no-default-features --features "${{ matrix.featureset }}"
@@ -141,13 +145,14 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        rust: [stable, beta, nightly, 1.39.0]
+        rust: [stable, beta, nightly, 1.40.0]
     steps:
-    - uses: actions/checkout@master
+    - uses: actions/checkout@main
     - uses: actions-rs/toolchain@v1
       with:
         toolchain: ${{ matrix.rust }}
         profile: minimal
+        override: true
     - name: Run tests
       uses: actions-rs/cargo@v1
       with:
@@ -162,11 +167,12 @@ jobs:
       matrix:
         os: [ubuntu-latest, windows-latest, macOS-latest]
     steps:
-    - uses: actions/checkout@master
+    - uses: actions/checkout@main
     - uses: actions-rs/toolchain@v1
       with:
         toolchain: stable
         profile: minimal
+        override: true
     - name: Run tests
       uses: actions-rs/cargo@v1
       with:
@@ -178,11 +184,12 @@ jobs:
     needs: check
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@master
+    - uses: actions/checkout@main
     - uses: actions-rs/toolchain@v1
       with:
         toolchain: stable
         profile: minimal
+        override: true
     - name: "Test log support"
       run: (cd tracing/test-log-support && cargo test)
     - name: "Test static max level"
@@ -196,12 +203,13 @@ jobs:
     needs: check
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@master
+    - uses: actions/checkout@main
     - uses: actions-rs/toolchain@v1
       with:
         toolchain: stable
         components: rustfmt
         profile: minimal
+        override: true
     - name: rustfmt
       uses: actions-rs/cargo@v1
       with:
@@ -212,7 +220,7 @@ jobs:
     # Check for any warnings. This is informational and thus is allowed to fail.
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@master
+    - uses: actions/checkout@main
     - uses: actions-rs/toolchain@v1
       with:
         toolchain: stable
@@ -227,7 +235,7 @@ jobs:
   cargo-audit:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@master
+    - uses: actions/checkout@main
     - name: Fetch latest release version of cargo-audit
       run: |
         mkdir -p .github/caching