chore: don't include dev-dependencies in MSRV builds at all

This commit removes the `--tests` from the MSRV build's `cargo check`
invocation. This way, CI will not try to compile dev dependencies on the
MSRV. This is fine, because the tests not building on 1.42.0 won't stop
1.42.0 users from depending on `tracing` crates.

Unfortunately, this means we have to remove the CI job that runs tests
on 1.42.0. But, a significant change in the behavior we're testing for
on our MSRV vs on stable seems unlikely.

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

.github/workflows/CI.yml

@@ -10,7 +10,6 @@ jobs:
   check-msrv:
     # Run `cargo check` on our minimum supported Rust version (1.42.0).
     runs-on: ubuntu-latest
-    rust: 1.42.0
     steps:
     - uses: actions/checkout@main
     - uses: actions-rs/toolchain@v1
@@ -22,12 +21,11 @@ jobs:
       uses: actions-rs/cargo@v1
       with:
         command: check
-        args: --all --bins --tests
+        args: --all --bins
 
   check:
     # Run `cargo check` first to ensure that the pushed code at least compiles.
     runs-on: ubuntu-latest
-    rust: stable
     steps:
     - uses: actions/checkout@main
     - uses: actions-rs/toolchain@v1
@@ -143,32 +141,6 @@ jobs:
         command: test
         args: --all
 
-  test-msrv:
-    # Test against the minimum supported Rust version
-    needs: check
-    runs-on: ubuntu-latest
-    steps:
-    - uses: actions/checkout@main
-    - uses: actions-rs/toolchain@v1
-      with:
-        toolchain: 1.42.0
-        profile: minimal
-        override: true
-    # We can't use `cargo test --all`, as that will also compile the examples.
-    # We don't guarantee the examples will work on the MSRV, because they have
-    # external dependencies which may not comply with our MSRV policy, but don't
-    # actually impact users on our MSRV.
-    - name: Run tests
-      uses: actions-rs/cargo@v1
-      with:
-        command: test
-        args: --workspace --lib --tests
-    - name: Run doctests
-      uses: actions-rs/cargo@v1
-      with:
-        command: test
-        args: --workspace --doc
-
   test-build-wasm:
     needs: check
     runs-on: ubuntu-latest