fix: add GOPATH/bin to PATH after go install shfmt

go install succeeds but $GOPATH/bin is not in PATH for subsequent steps
on ubuntu-latest runners. Appending to GITHUB_PATH makes shfmt available.

https://claude.ai/code/session_01KVdk4CmanaQvKXrk3inMoV

Author: claude

.github/workflows/ci.yml

@@ -15,7 +15,9 @@ jobs:
         run: sudo apt-get install -y shellcheck
 
       - name: Install shfmt
-        run: go install mvdan.cc/sh/v3/cmd/shfmt@latest
+        run: |
+          go install mvdan.cc/sh/v3/cmd/shfmt@latest
+          echo "$(go env GOPATH)/bin" >> "$GITHUB_PATH"
 
       - name: Check formatting (shfmt)
         run: shfmt -d setup/*.sh setup/install/*.sh