CI: Mark CGO warnings as errors

Treat warning as errors only in the CI. We can enforce it in the source
code (like setting CFLAGS in libcontainer/nsenter/nsenter.go), but that
can force other downstream to patch the code if thei C compiler produces
warnings. For that reason, we do it only on the CI.

Todays CGO warnings are quite hidden in the CI (only shown for the
compilation step, that is collapsed) and CI is green anyways. With this
patch, CI fails if a warning is introduced.

Signed-off-by: Rodrigo Campos <rodrigo@kinvolk.io>

.github/workflows/test.yml

@@ -11,6 +11,10 @@ on:
       - release-*
   pull_request:
 
+env:
+  # Don't ignore C warnings. Note that the output of "go env CGO_CFLAGS" by default is "-g -O2", so we keep them.
+  CGO_CFLAGS: -g -O2 -Werror
+
 jobs:
   test:
     runs-on: ubuntu-20.04

.github/workflows/validate.yml

@@ -25,6 +25,9 @@ jobs:
 
   compile-buildtags:
     runs-on: ubuntu-20.04
+    env:
+      # Don't ignore C warnings. Note that the output of "go env CGO_CFLAGS" by default is "-g -O2", so we keep them.
+      CGO_CFLAGS: -g -O2 -Werror
     steps:
       - uses: actions/checkout@v2
       - name: compile with no build tags