ci: use self-built mcpp in ARM source builds

Author: wellwei

.github/workflows/ci-aarch64-fresh-install.yml

@@ -127,14 +127,14 @@ jobs:
           # musl-static target is the published path, so build with --target.
           git clone --depth 1 https://github.com/mcpp-community/mcpp /tmp/mcpp-src
           cd /tmp/mcpp-src
-          mcpp self config --mirror GLOBAL 2>/dev/null || true
-          mcpp build --target aarch64-linux-musl
+          "$MCPP_SELF" self config --mirror GLOBAL 2>/dev/null || true
+          "$MCPP_SELF" build --target aarch64-linux-musl
           m=$(find target/aarch64-linux-musl -type f -name mcpp | head -1)
           file "$m" | grep -q "ARM aarch64" || { echo "expected aarch64 mcpp"; exit 1; }
           "$m" --version
           git clone --depth 1 https://github.com/openxlings/xlings /tmp/xlings-src
           cd /tmp/xlings-src
-          mcpp build --target aarch64-linux-musl
+          "$MCPP_SELF" build --target aarch64-linux-musl
           x=$(find target/aarch64-linux-musl -type f -name xlings | head -1)
           file "$x" | grep -q "ARM aarch64" || { echo "expected aarch64 xlings"; exit 1; }
           "$x" --version