fix: ci.yml — install musl-gcc before setting default

The musl-gcc test step assumed the toolchain was already cached.
After cache clears, `toolchain default gcc@15.1.0-musl` fails with
"not installed". Add explicit install before setting default.

Author: Sunrisepeak

.github/workflows/ci.yml

@@ -147,6 +147,7 @@ jobs:
       - name: "Toolchain: musl-gcc — mcpp new → build → run"
         run: |
           MCPP=$(realpath "$(find target -type f -name mcpp -printf '%T@ %p\n' | sort -rn | head -1 | cut -d' ' -f2)")
+          "$MCPP" toolchain install gcc 15.1.0-musl
           "$MCPP" toolchain default gcc@15.1.0-musl
           TMP=$(mktemp -d)
           cd "$TMP"