Merge #18910

18910: CI: add bors.toml r=kaspar030 a=kaspar030

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### Contribution description

This adds configuration for [bors](https://bors.tech).

This has been discussed in CI meetings. The intended workflow is:

1. regular PR commits get built by CI as usual, but doing a quick-build (building only a subset of all boards), unless "CI: full build" is set
2. once that passes and the PR is ACKed, instead of merging using the merge button, we use bors to merge by typing "bors merge".
3. bors takes the PR, pushes it to the "testing" branch
4. CI does a full build of "testing"
5. on success, bors merges testing into master

The main benefits are

1. a quickbuild / full build scheme ("regular" PR builds will be <5 minutes, but only fully built branches get merged into master)
2. bors does "rollups", meaning, if multiple PRs are to be "bors merged", bors merges them together, tests them together and merges them together. That's more efficient than doing this sequentially, and prevents semantic merge conflicts (e.g., PR a and b pass CI individually, but fail when both merged, breaking master).

Once fully implemented, using bors will be a workflow change (no more "pressing the green button", but typing "bors merge" instead). But we've been using bors for RIOT-OS/riotdocker for a long time now, the experience was good (as opposed to disruptive).

This PR just adds the necessary bors configuration, and once merged will allow using bors to merge PRs using "bors merge", but the previous workflow is still in place (manual merging can be done, and "regular" PR builds are still full builds).

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### Testing procedure

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### Issues/PRs references

bors tracking issue: #18911 
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Co-authored-by: Kaspar Schleiser <kaspar@schleiser.de>

.github/labeler.yml

@@ -20,6 +20,7 @@
   - ".github/**/*.yml"
   - ".murdock"
   - ".murdock.yml"
+  - "bors.toml"
 
 "Area: CoAP":
   - "sys/net/application_layer/*coap/**/*"

.github/workflows/static-test.yml

@@ -4,6 +4,8 @@ on:
   push:
     branches:
       - master
+      - staging
+      - trying
     tags:
       - '[0-9][0-9][0-9][0-9].[0-9][0-9]-RC[0-9]*'
       - '[0-9][0-9][0-9][0-9].[0-9][0-9]'

.github/workflows/tools-buildtest.yml

@@ -4,6 +4,8 @@ on:
   push:
     branches:
       - master
+      - staging
+      - trying
     tags:
       - '*'
   pull_request:

bors.toml

@@ -0,0 +1,35 @@
+# List of commit statuses that must pass on the PR commit when it is r+-ed.
+# (listed in same order as in the repo's branch protection settings)
+pr_status = [
+ "python-tests",
+ "tools-build",
+ "check-labels",
+ "static-tests",
+ "check-commits (commit-msg)",
+ "check-commits (pr_check)",
+ "Murdock",
+ ]
+
+# List of commit statuses that must pass on the merge commit before it is
+# pushed to master.
+status = [
+  "Murdock",
+  "static-tests",
+  "tools-build",
+  ]
+
+# Number of project members who must approve the PR (using GitHub Reviews)
+# before it is pushed to master.
+# This necessary even with the check-labels action (which checks for >1 ACKs),
+# as that passes when there's no review or no special label set.
+required_approvals = 1
+
+# delete branch after successful merge
+delete_merged_branches = true
+
+# timeout for bors-initiated builds.
+# unfortunately bors is not the only entity queuing builds, so this needs
+# to take a full nightly build *plus* another full bors build *plus* some
+# possible intermediate PR builds into accound.
+# let's try ten hours (36000s)
+timeout_sec = 36000