fix: add GOTOOLCHAIN=local + upload build.log artifact, downgrade go.mod to 1.21

the makepkg exit code 4 after run #39 means 'user function error'
(build/package failed). all go files pass gofmt individually,
so ruling out syntax. adding GOTOOLCHAIN=local prevents any
toolchain download attempt, and the build log artifact lets
us see the actual error without being signed into github.

Author: MochiOS Team

.github/workflows/build.yml

@@ -68,6 +68,7 @@ jobs:
           export WORK_DIR="$PWD/work"
           export GNUPGHOME="/home/mochi/.mochios-gnupg"
           export SIGN_KEY="ci@mochios.dev"
+          export GOTOOLCHAIN="local"
           bash build.sh 2>&1 | tee build.log; exit ${PIPESTATUS[0]}
 
       - name: Upload build log
@@ -80,6 +81,14 @@ jobs:
             echo "No build log found or empty"
           fi
 
+      - name: Upload build log as artifact
+        if: always()
+        uses: actions/upload-artifact@v4
+        with:
+          name: build-log
+          path: build.log
+          retention-days: 7
+
       - name: List ISO output
         run: |
           ls -lh out/*.iso 2>/dev/null || echo "No ISO found"

mochi/go.mod

@@ -1,3 +1,3 @@
 module mochi
 
-go 1.26
+go 1.21