fix(install-e2e): REPO_DIR -> REPO_ROOT in macos dmg driver self-check

The get-url shim self-check referenced REPO_DIR, which is never defined
(the script defines REPO_ROOT). Under set -u the stage phase dies
immediately, so every macos desktop-installer@latest leg fails before
install. Introduced in 368164d7 with the shim itself.

Verified: bash -n clean; shim + self-check block replayed standalone
with the fix and passes.

tests/install/macos-desktop-e2e.sh

@@ -145,7 +145,7 @@ EOF
   export PATH="$SHIM_DIR:$PATH"
 
   # check it worked
-  observed_git_url="$(git -C "$REPO_DIR" remote get-url origin)"
+  observed_git_url="$(git -C "$REPO_ROOT" remote get-url origin)"
   if [[ "$observed_git_url" != "$REPO_URL_HTTPS" ]]; then
     fail "failed git remote get-url shim: origin resolves to '$observed_git_url', expected '$REPO_URL_HTTPS'"
   fi