ci: install pnpm into a per-job dir to fix the host-runner setup race

`pnpm/action-setup@v4` defaults to installing pnpm in the shared
`~/setup-pnpm`. On the single host-mode runner $HOME is shared across
concurrent jobs, so when two pnpm jobs overlap (e.g. a post-merge
`dev-deploy` and `ui-test`, which sit in different concurrency groups)
their self-installers race and one fails with
`ENOTEMPTY ... rmdir '~/setup-pnpm/node_modules/.bin/store/v11/files'`
before the tests even run.

Point each step's `dest` at `${{ runner.temp }}/setup-pnpm` (a per-job
isolated directory) so concurrent jobs never share the install location.
The action still adds `dest` to PATH, so setup-node's pnpm cache and
later `pnpm` calls are unaffected; the pnpm package store stays shared
(safe — pnpm locks it). Applied to the three workflows that set up pnpm:
ui-test, dev-deploy, prod-build.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitea/workflows/prod-build.yaml

@@ -43,6 +43,11 @@ jobs:
         uses: pnpm/action-setup@v4
         with:
           version: 11.0.7
+          # Install pnpm into a per-job directory so concurrent jobs on
+          # the shared host runner do not race on the default
+          # `~/setup-pnpm` (the self-installer otherwise fails with
+          # `ENOTEMPTY` while cleaning a sibling job's install).
+          dest: ${{ runner.temp }}/setup-pnpm
 
       - name: Set up Node
         uses: actions/setup-node@v4