ci: migrate CI jobs from depot to ARC self-hosted runners

Switch every job in the main CI workflow off of the
depot-ubuntu-24.04 runner family and onto the org-wide ARC
self-hosted pool registered against the lightninglabs org. All
six jobs (commit-message, static-checks, lint, cross-compile,
unit-test, systest) now use the [self-hosted] label, which is
applied to every runner registered by our actions-runner-controller
deployment in the staging cluster.

Depot exposes sized runner classes (24.04, 24.04-4, 24.04-8) but
the current ARC RunnerDeployment provisions a single uniform pool
with no size dimension, so every job collapses onto the same
[self-hosted] label. We are intentionally not introducing custom
labels per job class yet; if a job turns out to need dedicated
hardware we can add a label on the RunnerDeployment side and key
that job to it, rather than carrying depot's sizing taxonomy
forward.

Existing depot CI runs on main and recent PRs serve as the
baseline for a head-to-head comparison against the new pool, so
no parallel benchmarking workflow is added in this commit.

Author: Roasbeef

.github/workflows/main.yml

@@ -36,7 +36,7 @@ jobs:
   ########################
   commit-message:
     name: Commit Message
-    runs-on: depot-ubuntu-24.04
+    runs-on: [self-hosted]
     steps:
       - name: Git checkout
         uses: actions/checkout@v5
@@ -67,7 +67,7 @@ jobs:
   ########################
   static-checks:
     name: Static Checks
-    runs-on: depot-ubuntu-24.04
+    runs-on: [self-hosted]
     steps:
       - name: Git checkout
         uses: actions/checkout@v5
@@ -122,7 +122,7 @@ jobs:
   ########################
   lint:
     name: Lint code
-    runs-on: depot-ubuntu-24.04-8
+    runs-on: [self-hosted]
     steps:
       - name: git checkout
         uses: actions/checkout@v5
@@ -147,7 +147,7 @@ jobs:
   ########################
   cross-compile:
     name: Cross compilation
-    runs-on: depot-ubuntu-24.04-4
+    runs-on: [self-hosted]
     strategy:
       fail-fast: true
       matrix:
@@ -186,7 +186,7 @@ jobs:
   ########################
   unit-test:
     name: Run unit tests
-    runs-on: depot-ubuntu-24.04-4
+    runs-on: [self-hosted]
     strategy:
       # Allow other tests in the matrix to continue if one fails.
       fail-fast: false
@@ -265,7 +265,7 @@ jobs:
   ########################
   systest:
     name: Run system tests
-    runs-on: depot-ubuntu-24.04-8
+    runs-on: [self-hosted]
     strategy:
       # Allow other tests in the matrix to continue if one fails.
       fail-fast: false