build: Add CI check for CGO_ENABLED=0

Some applications that depend on go-git may not need
to build using CGO. The new test ensures no new
regressions going forwards.

Signed-off-by: Paulo Gomes <pjbgf@linux.com>

.github/workflows/git.yml

@@ -38,3 +38,8 @@ jobs:
  
     - name: Test
       run: make test-coverage
+
+    - name: Build go-git with CGO disabled
+      run: go build ./...
+      env:
+        CGO_ENABLED: 0