check MSRV in CI

It's very easy to use new rust features without realizing it since
the CI and local development workflows may use the latest rust version.
We try to keep some backwards compatibility with rust versions to make
packaging easier for some OS-level package-managers like Void Linux's.
See helix-editor#1881.

This change runs the "Check" step for the pinned version of rust in
the rust-toolchain.toml file as well as the MSRV version in a matrix.
In order to bump the MSRV, we need to edit

    .github/workflows/msrv-rust-toolchain.toml

This commit sets the MSRV as 1.60.0 but a later child commit will
reduce the MSRV back to 1.57.0.

Closes helix-editor#2482.

.github/workflows/build.yml

@@ -11,10 +11,17 @@ jobs:
   check:
     name: Check
     runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        rust: [stable, msrv]
     steps:
       - name: Checkout sources
         uses: actions/checkout@v3
 
+      - name: Use MSRV rust toolchain
+        if: matrix.rust == 'msrv'
+        run: cp .github/workflows/msrv-rust-toolchain.toml rust-toolchain.toml
+
       - name: Install stable toolchain
         uses: helix-editor/rust-toolchain@v1
         with:
@@ -50,8 +57,8 @@ jobs:
         uses: actions/cache@v3
         with:
           path: runtime/grammars
-          key: ${{ runner.os }}-v2-tree-sitter-grammars-${{ hashFiles('languages.toml') }}
-          restore-keys: ${{ runner.os }}-v2-tree-sitter-grammars-
+          key: ${{ runner.os }}-stable-v${{ env.CACHE_VERSION }}-tree-sitter-grammars-${{ hashFiles('languages.toml') }}
+          restore-keys: ${{ runner.os }}-stable-v${{ env.CACHE_VERSION }}-tree-sitter-grammars-
 
       - name: Run cargo test
         uses: actions-rs/cargo@v1

.github/workflows/msrv-rust-toolchain.toml

@@ -0,0 +1,3 @@
+[toolchain]
+channel = "1.60.0"
+components = ["rustfmt", "rust-src"]