fix(buildkite): Catch '?' yaml syntax for plugins (#15666)

lib/modules/manager/buildkite/__fixtures__/pipeline8.yml

@@ -0,0 +1,4 @@
+steps:
+  - plugins:
+      - ? ssh://git@github.company.com/some-org/some-plugin.git#v3.2.7
+        : enforce_peer_deps: false

lib/modules/manager/buildkite/extract.spec.ts

@@ -60,5 +60,16 @@ describe('modules/manager/buildkite/extract', () => {
       };
       expect(res).toEqual([expectedPackageDependency]);
     });
+
+    it('extracts plugin with preceding ?', () => {
+      const res = extractPackageFile(Fixtures.get('pipeline8.yml'))?.deps;
+      const expectedPackageDependency: PackageDependency = {
+        currentValue: 'v3.2.7',
+        datasource: 'github-tags',
+        depName: 'some-org/some-plugin',
+        registryUrls: ['https://github.company.com'],
+      };
+      expect(res).toEqual([expectedPackageDependency]);
+    });
   });
 });

lib/modules/manager/buildkite/extract.ts

@@ -13,7 +13,7 @@ export function extractPackageFile(content: string): PackageFile | null {
     for (const line of lines) {
       // Search each line for plugin names
       const depLineMatch = regEx(
-        /^[\s-]*(?<depName>[^#\s]+)#(?<currentValue>[^:]+)/
+        /^\s*(?:-\s+(?:\?\s+)?)?(?<depName>[^#\s]+)#(?<currentValue>[^:]+)/
       ).exec(line);
 
       if (depLineMatch?.groups) {