CI/CD: Update Clippy and MSRV testing from *ring*.

.github/workflows/ci.yml

@@ -9,23 +9,16 @@ jobs:
 
     runs-on: ubuntu-18.04
 
-    strategy:
-      matrix:
-        rust_channel:
-          # MSRV
-          - 1.37.0
-          - stable
-
     steps:
       - uses: actions-rs/toolchain@v1
         with:
-          toolchain: ${{ matrix.rust_channel }}
+          toolchain: stable
           profile: minimal
           components: clippy
 
       - uses: actions/checkout@v2
 
-      - run: cargo +${{ matrix.rust_channel }} clippy --all-features ---all-targets -- --deny warnings
+      - run: cargo clippy --all-features ---all-targets -- --deny warnings
 
   audit:
     # Don't run duplicate `push` jobs for the repo owner's PRs.
@@ -145,14 +138,43 @@ jobs:
 
         rust_channel:
           - stable
-          - beta
           - nightly
+          - 1.37.0 # MSRV
+          - beta
 
         exclude:
           # The stable channel doesn't have aarch64-apple-darwin support yet.
           - target: aarch64-apple-darwin
             rust_channel: stable
 
+          # The MSRV channel doesn't have aarch64-apple-darwin support yet.
+          - target: aarch64-apple-darwin
+            rust_channel: 1.37.0
+
+          # Only do MSRV testing on release builds.
+          - mode: # debug
+            rust_channel: 1.37.0
+
+          # 1.37.0 doesn't support `-Clink-self-contained`.
+          - target: aarch64-unknown-linux-musl
+            rust_channel: 1.37.0
+
+          # 1.37.0 doesn't support `-Clink-self-contained`.
+          - target: armv7-unknown-linux-musleabihf
+            rust_channel: 1.37.0
+
+          # 1.37.0 doesn't support `-Clink-self-contained`.
+          - target: i686-unknown-linux-musl
+            rust_channel: 1.37.0
+
+          # 1.37.0 doesn't support `-Clink-self-contained`.
+          - target: x86_64-unknown-linux-musl
+            rust_channel: 1.37.0
+
+          # https://github.com/rust-lang/rust/pull/67429
+          - target: x86_64-pc-windows-gnu
+            rust_channel: 1.37.0
+
         include:
           - target: aarch64-apple-darwin
             # macos-latest didn't work.