fix(action): workaround for private repos

Download repo tarball instead of trying to use git switch to the proper
branch.

action.yml

@@ -7,7 +7,8 @@ description: |
   You can post this report as a comment to the PR associated with the current
   branch and/or export the report as a markdown artifact.
 
-  Requires `nix`, `jq`, `curl`, `sed` and `coreutils` to be in path.
+  Requires `nix`, `jq`, `curl`, `sed`, `gunzip`, `tar` and `coreutils` to be in
+  the runner's path.
 
   Usage:
 
@@ -98,10 +99,15 @@ runs:
         }
 
         if [ "$PRIVATE_REPO" = 'true' ]; then 
-          log "In a private repo, switching to $HEAD_BRANCH to build the old report"
-          git switch "$HEAD_BRANCH"
+          log "In a private repo, downloading $HEAD_BRANCH to build the old report"
 
-          "$GITHUB_ACTION_PATH/create-report.sh" > old-report.json
+          mkdir -p old
+          curl -X 'GET' \
+            "$GITHUB_API_URL/repos/$GITHUB_REPOSITORY/archive/$HEAD_BRANCH.tar.gz" \
+            -H "Authorization: token $GITHUB_TOKEN" |
+              tar -zvx --strip-components=1 -C old
+
+          (cd old && "$GITHUB_ACTION_PATH/create-report.sh") > old-report.json
 
           log "Reporting on sizes and comparing to sizes in $HEAD_BRANCH"
           "$GITHUB_ACTION_PATH/comment_on_pr.sh" report.json old-report.json