ci: fix the codegen workflow

The codegen workflow recently sometimes failed to detect files modified
by codegen. This has been analysed in a test PR:
https://github.com/rook/rook/pull/14723

The result of the analysis is the insight that the codegen tooling did not work
as expcted because the git checkout was not located in the src/github.com
directory under the GOPATH.

This change fixes the codegen workflow by post-processing the git checkout
action by copying the git checkout directory to the expected location.

Co-authored-by: Travis Nielsen <tnielsen@redhat.com>
Signed-off-by: Michael Adam <obnox@samba.org>
(cherry picked from commit f32d3df995)

.github/workflows/codegen.yml

@@ -38,8 +38,20 @@ jobs:
         with:
           go-version: "1.22"
 
+      - name: post-process checkout - copy to github.com
+        run: |
+          # copy to the github.com path so codegen will work as expected
+          mkdir -p $(go env GOPATH)/src/github.com/rook/rook/
+          cp -R . $(go env GOPATH)/src/github.com/rook/rook/
+
       - name: run codegen
-        run: GOPATH=$(go env GOPATH) make codegen
+        run: |
+          cd $(go env GOPATH)/src/github.com/rook/rook
+          GOPATH=$(go env GOPATH) make codegen
 
       - name: validate codegen
-        run: tests/scripts/validate_modified_files.sh codegen
+        run: |
+          # run validation again, explicitly.
+          # it is also part of 'make codegen'
+          cd $(go env GOPATH)/src/github.com/rook/rook
+          tests/scripts/validate_modified_files.sh codegen