CI: Make shellcheck a standalone pipeline (#55082)

* Make shellcheck standalone pipeline

* Run shellcheck on every .sh change

* Change base image

* Trigger on script changes

* Run shellcheck on PRs only

scripts/drone/pipelines/shellcheck.star

@@ -0,0 +1,48 @@
+load(
+    'scripts/drone/steps/lib.star',
+    'build_image',
+    'compile_build_cmd'
+)
+
+load(
+    'scripts/drone/utils/utils.star',
+    'pipeline',
+)
+
+trigger = {
+    'event': [
+        'pull_request',
+    ],
+    'paths': {
+        'exclude': [
+            '*.md',
+            'docs/**',
+            'latest.json',
+        ],
+        'include': [
+            'scripts/**/*.sh'
+        ],
+    },
+}
+
+def shellcheck_step():
+    return {
+        'name': 'shellcheck',
+        'image': build_image,
+        'depends_on': [
+            'compile-build-cmd',
+        ],
+        'commands': [
+            './bin/build shellcheck',
+        ],
+    }
+
+def shellcheck_pipeline():
+    steps = [
+        compile_build_cmd(),
+        shellcheck_step(),
+    ]
+    return pipeline(
+            name='pr-shellcheck', edition="oss", trigger=trigger, services=[], steps=steps,
+    )
+

scripts/drone/pipelines/test_backend.star

@@ -4,7 +4,6 @@ load(
     'download_grabpl_step',
     'wire_install_step',
     'codespell_step',
-    'shellcheck_step',
     'lint_backend_step',
     'lint_drone_step',
     'test_backend_step',
@@ -28,7 +27,6 @@ def test_backend(trigger, ver_mode):
     ]
     test_steps = [
         codespell_step(),
-        shellcheck_step(),
         lint_backend_step(edition="oss"),
         test_backend_step(edition="oss"),
         test_backend_integration_step(edition="oss"),