github: Split pre/post-submit

For now they're identical but they may start to diverge some.

Bug: b/338083578
Change-Id: I28b32b797d518e128f79cbf98fe16c47f38d8875
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/quickstart/bazel/+/210115
Reviewed-by: Anthony DiGirolamo <tonymd@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed-service-accounts.iam.gserviceaccount.com>
Pigweed-Auto-Submit: Rob Mohr <mohrr@google.com>
Presubmit-Verified: CQ Bot Account <pigweed-scoped@luci-project-accounts.iam.gserviceaccount.com>
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>

.github/workflows/postsubmit.yaml

diff --git a/.github/workflows/basic.yaml b/.github/workflows/postsubmit.yaml
similarity index 90%
copy from .github/workflows/basic.yaml
copy to .github/workflows/postsubmit.yaml
index 57f0b58..0edaf94 100644
--- a/.github/workflows/basic.yaml
+++ b/.github/workflows/postsubmit.yaml
@@ -1,6 +1,9 @@
 name: basic
-run-name: presubmit run triggered by ${{ github.actor }}
-on: [push]
+run-name: postsubmit run
+
+on:
+  push
+
 jobs:
   bazel-build-test:
     runs-on: ubuntu-latest

.github/workflows/presubmit.yaml

diff --git a/.github/workflows/basic.yaml b/.github/workflows/presubmit.yaml
similarity index 91%
rename from .github/workflows/basic.yaml
rename to .github/workflows/presubmit.yaml
index 57f0b58..e9aa5c0 100644
--- a/.github/workflows/basic.yaml
+++ b/.github/workflows/presubmit.yaml
@@ -1,6 +1,10 @@
 name: basic
 run-name: presubmit run triggered by ${{ github.actor }}
-on: [push]
+
+on:
+  pull_request:
+    types: [opened, synchronize, reopened]
+
 jobs:
   bazel-build-test:
     runs-on: ubuntu-latest