chore(ci): Shot-in-the-dark fix for MSRV updating

With rust-lang#13254, we found MSRV updating is broken.
PR rust-lang#12775 is the last MSRV we got.
That was merged before rust-lang#12654 and rust-lang#13106.
That makes rust-lang#12654 the most likely culprit.

Looking at the logs:
```
DEBUG: Matched 24 file(s) for manager regex: Cargo.toml, benches/benchsuite/Cargo.toml, benches/capture/Cargo.toml, crates/cargo-platform/Cargo.toml, crates/cargo-test-macro/Cargo.toml, crates/cargo-test-support/Cargo.toml, crates/cargo-test-support/containers/apache/bar/Cargo.toml, crates/cargo-test-support/containers/sshd/bar/Cargo.toml, crates/cargo-util-schemas/Cargo.toml, crates/cargo-util/Cargo.toml, crates/crates-io/Cargo.toml, crates/home/Cargo.toml, crates/mdman/Cargo.toml, crates/resolver-tests/Cargo.toml, crates/rustfix/Cargo.toml, crates/semver-check/Cargo.toml, crates/xtask-build-man/Cargo.toml, crates/xtask-bump-check/Cargo.toml, crates/xtask-stale-label/Cargo.toml, credential/cargo-credential-1password/Cargo.toml, credential/cargo-credential-libsecret/Cargo.toml, credential/cargo-credential-macos-keychain/Cargo.toml, credential/cargo-credential-wincred/Cargo.toml, credential/cargo-credential/Cargo.toml
...
DEBUG: manager extract durations (ms)
{
  "managers": {
    "dockerfile": 30,
    "github-actions": 38,
    "regex": 386,
    "cargo": 855
  }
}
DEBUG: Found cargo package files
DEBUG: Found dockerfile package files
DEBUG: Found github-actions package files
DEBUG: Found 25 package file(s)
```
Our regex managers have the files matched
but no regex manager packages are found.
I think this means that the name association failed or the regex within
the file content failed.

The differences between cargo and my other projects
- Use of `:`
- `depNameTemplate`
- Presence of `\b`

As a first attempt, I'm going to switch `\b` to `\\b` to be like the
other escaped regex values.

.github/renovate.json5

@@ -15,8 +15,8 @@
         'Cargo.toml$',
       ],
       matchStrings: [
-        '\bMSRV:1\b.*?(?<currentValue>\\d+\\.\\d+(\\.\\d+)?)',
-        '(?<currentValue>\\d+\\.\\d+(\\.\\d+)?).*?\bMSRV:1\b',
+        '\\bMSRV:1\\b.*?(?<currentValue>\\d+\\.\\d+(\\.\\d+)?)',
+        '(?<currentValue>\\d+\\.\\d+(\\.\\d+)?).*?\\bMSRV:1\\b',
       ],
       depNameTemplate: 'MSRV:1',  // Support 1 version of rustc
       packageNameTemplate: 'rust-lang/rust',