test: iofail and merge_group trigger (#224)

This PR makes two small changes to CI:
- Adds a `merge_group` trigger 
- Passes the `--iofail` option to `build-args`

The motivation of having the `merge_group` trigger is to ensure that
there is a final build that happens before a PR is merged into `main`,
preventing situations where a release happens while a PR is pending and
then only failing after being merged.

For this to take effect, I need someone with adequate permissions to
require merge queues for the `main` branch. These should still be
configured as squash commits. I would be fine setting the merge limit
low (maybe even 1) for simplicity since we don't yet have high enough
volume of commits for this to matter.

The `--iofail` option is straightforward, mirroring what Mathlib already
does.

Author: chenson2018

.github/workflows/docs.yml

@@ -25,7 +25,7 @@ jobs:
         id: build-lean
         uses: leanprover/lean-action@v1
         with:
-          build-args: "--wfail"
+          build-args: "--wfail --iofail"
 
       - name: Build project documentation
         id: build-docgen

.github/workflows/lean_action_ci.yml

@@ -6,6 +6,7 @@ on:
       - main
       - nightly-testing
   pull_request:
+  merge_group:
   workflow_dispatch:
 
 jobs:
@@ -15,7 +16,7 @@ jobs:
       - uses: actions/checkout@v4
       - uses: leanprover/lean-action@v1
         with:
-          build-args: "--wfail"
+          build-args: "--wfail --iofail"
           mk_all-check: "true"
       - name: "lake exe shake"
         run: |