ci: Use is-terminal 0.4.7 for MSRV builds.

is-terminal 0.4.8 updated its MSRV to 1.63, so we can't use it
with our MSRV of 1.56. Force usage of the older version which has
an older MSRV.

.github/workflows/ci.yml

@@ -36,6 +36,9 @@ jobs:
         with:
           toolchain: ${{ matrix.rust }}
           components: clippy
+      - name: Force older version of is-terminal for MSRV builds
+        if: matrix.rust == '1.56.0'
+        run:  cargo update -p is-terminal --precise 0.4.7
       - name: Clippy
         run: cargo clippy --all -- -D warnings
       - name: Run tests