pkg/runner: add support to replace GitHub's env (#1197)

There might be use cases where users want to use GitHub's variables in
the environment variables, which is a valid use case.

This commits adds support for replacement of GitHub's env with GitHub's
values.

Signed-off-by: André Martins <aanm90@gmail.com>

pkg/runner/runner_test.go

@@ -147,6 +147,7 @@ func TestRunEvent(t *testing.T) {
 		{workdir, "evalmatrixneeds2", "push", "", platforms},
 		{workdir, "evalmatrix-merge-map", "push", "", platforms},
 		{workdir, "evalmatrix-merge-array", "push", "", platforms},
+		{workdir, "issue-1195", "push", "", platforms},
 
 		{workdir, "basic", "push", "", platforms},
 		{workdir, "fail", "push", "exit with `FAILURE`: 1", platforms},