chore: remove deprecated add-path from CI

## Motivation

Use of `echo "::add-path::<PATH>"` has been deprecated on GitHub CI.

## Solution

Update CI to use new path setter `echo "<PATH>" >> $GITHUB_PATH`.

.github/workflows/CI.yml

@@ -82,7 +82,7 @@ jobs:
       with:
         path: ${{ runner.tool_cache }}/cargo-hack/bin
         key: cargo-hack-bin-${{ hashFiles('.github/caching/cargo-hack.lock') }}
-    - run: echo "::add-path::${{ runner.tool_cache }}/cargo-hack/bin"
+    - run: echo "${{ runner.tool_cache }}/cargo-hack/bin" >> $GITHUB_PATH
     # if `cargo-hack` somehow doesn't exist after loading it from the cache,
     # make *sure* it's there.
     - run: cargo hack --help || { cargo install --force cargo-hack; }
@@ -303,5 +303,5 @@ jobs:
       with:
         command: install
         args: --root ${{ runner.tool_cache }}/cargo-audit --force cargo-audit
-    - run: echo "::add-path::${{ runner.tool_cache }}/cargo-audit/bin"
+    - run: echo "${{ runner.tool_cache }}/cargo-audit/bin" >> $GITHUB_PATH
     - run: cargo audit