build: Simplify/improve verify-vendor.sh

`verify-vendor.sh` compares the content of the `vendor` directory after
running `make vendor` with a copy which was made before running `make
vendor`.
This does not catch changes to go.mod/go.sum made by `go mod tidy` which
we forgot to commit.

This commit uses `git diff --exit-code` instead of running `diff` against the
vendor directory.

This will catch all uncommitted changes, which should be fine when `make
vendorcheck` is run.

Using `git diff` instead of `diff` allows to remove quite some code from
`verify-vendor.sh`

This fixes crc-org#4133

verify-vendor.sh

@@ -6,39 +6,22 @@ set -o pipefail
 
 export GO111MODULE=on
 
-readonly REPO_ROOT_DIR="$(git rev-parse --show-toplevel 2> /dev/null)"
-readonly TMP_DIFFROOT="$(mktemp -d "${REPO_ROOT_DIR}"/tmpdiffroot.XXXXXX)"
-
-cleanup() {
-  rm -rf "${TMP_DIFFROOT}"
-}
-
-trap "cleanup" EXIT SIGINT
-
-cleanup
-
 if [[ -n $(git status -s vendor/) ]]; then
         echo 'vendor/ directory has uncommitted changes, please check `git status vendor`'
         exit 1
 fi
 
-mkdir "${TMP_DIFFROOT}"
-cp -aR "${REPO_ROOT_DIR}/vendor" "${TMP_DIFFROOT}"
-
 make vendor
 
-echo "Diffing ${REPO_ROOT_DIR} against freshly generated codegen"
-ret=0
-diff --strip-trailing-cr -Naupr "${REPO_ROOT_DIR}/vendor" "${TMP_DIFFROOT}/vendor" || ret=1
+go mod verify
 
-# Restore working tree state
-rm -fr "${REPO_ROOT_DIR}/vendor"
-cp -aR "${TMP_DIFFROOT}"/* "${REPO_ROOT_DIR}"
+echo "Diffing $(pwd)"
+git diff --exit-code vendor go.mod go.sum
 
-if [[ $ret -eq 0 ]]
+if [[ $? -eq 0 ]]
 then
-  echo "${REPO_ROOT_DIR} up to date."
+  echo "$(pwd) up to date."
 else
-  echo "${REPO_ROOT_DIR} is out of date. Please run make vendor"
+  echo "$(pwd) is out of date. Please run make vendor"
   exit 1
 fi