CI: Use an env var for MSRV

This just reduces duplication in the build workflow - no functional
change - to make updating the MSRV easier in the future.

.github/workflows/build.yml

@@ -8,6 +8,9 @@ on:
   schedule:
     - cron: "00 01 * * *"
 
+env:
+  MSRV: "1.76"
+
 jobs:
   check:
     name: Check (msrv)
@@ -18,7 +21,9 @@ jobs:
         uses: actions/checkout@v4
 
       - name: Install MSRV toolchain
-        uses: dtolnay/rust-toolchain@1.76
+        uses: dtolnay/rust-toolchain@master
+        with:
+          toolchain: ${{ env.MSRV }}
 
       - uses: Swatinem/rust-cache@v2
         with:
@@ -39,7 +44,9 @@ jobs:
         uses: actions/checkout@v4
 
       - name: Install MSRV toolchain
-        uses: dtolnay/rust-toolchain@1.76
+        uses: dtolnay/rust-toolchain@master
+        with:
+          toolchain: ${{ env.MSRV }}
 
       - uses: Swatinem/rust-cache@v2
         with:
@@ -71,8 +78,9 @@ jobs:
         uses: actions/checkout@v4
 
       - name: Install MSRV toolchain
-        uses: dtolnay/rust-toolchain@1.76
+        uses: dtolnay/rust-toolchain@master
         with:
+          toolchain: ${{ env.MSRV }}
           components: rustfmt, clippy
 
       - uses: Swatinem/rust-cache@v2
@@ -99,7 +107,9 @@ jobs:
         uses: actions/checkout@v4
 
       - name: Install MSRV toolchain
-        uses: dtolnay/rust-toolchain@1.76
+        uses: dtolnay/rust-toolchain@master
+        with:
+          toolchain: ${{ env.MSRV }}
 
       - uses: Swatinem/rust-cache@v2
         with: