Workaround bug in gh cli (#3338)

Add workaround for https://github.com/cli/cli/issues/1820 to our
integration scripting. There's something broken about detection for the
appropriate HEAD ref for the PR, so we set it explicitly.
diff --git a/scripts/git/google_to_main.sh b/scripts/git/google_to_main.sh
index 8b8543d..b00fd9a 100755
--- a/scripts/git/google_to_main.sh
+++ b/scripts/git/google_to_main.sh
@@ -56,4 +56,7 @@
   echo "${BODY?}"
   exit 1
 fi
-gh pr create --base main --title="${TITLE?}" --body="${BODY?}"
+
+# Workaround https://github.com/cli/cli/issues/1820
+GITHUB_USERNAME="$(gh config get -h github.com user)"
+gh pr create --base main --head="${GITHUB_USERNAME?}:${PR_BRANCH?}" --title="${TITLE?}" --body="${BODY?}"
diff --git a/scripts/git/main_to_google.sh b/scripts/git/main_to_google.sh
index ee07573..b9e56ae 100755
--- a/scripts/git/main_to_google.sh
+++ b/scripts/git/main_to_google.sh
@@ -56,4 +56,7 @@
   echo "${BODY?}"
   exit 1
 fi
-gh pr create --base google --title="${TITLE?}" --body="${BODY?}"
+
+# Workaround https://github.com/cli/cli/issues/1820
+GITHUB_USERNAME="$(gh config get -h github.com user)"
+gh pr create --base google --head="${GITHUB_USERNAME?}:${PR_BRANCH?}" --title="${TITLE?}" --body="${BODY?}"
diff --git a/scripts/git/update_tf_submodule.sh b/scripts/git/update_tf_submodule.sh
index 0031cfa..28799d1 100755
--- a/scripts/git/update_tf_submodule.sh
+++ b/scripts/git/update_tf_submodule.sh
@@ -72,4 +72,7 @@
   echo "${BODY?}"
   exit 1
 fi
-gh pr create --title="${TITLE?}" --body="${BODY?}" --base="${BASE_BRANCH?}"
+
+# Workaround https://github.com/cli/cli/issues/1820
+GITHUB_USERNAME="$(gh config get -h github.com user)"
+gh pr create --base="${BASE_BRANCH?}" --head="${GITHUB_USERNAME?}:${PR_BRANCH?}" --title="${TITLE?}" --body="${BODY?}"