chore(prlint): get sha from context (#25457)

The `sha` doesn't exist in some of the payloads, but should always exist in the event context.

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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: corymhall

tools/@aws-cdk/prlint/index.ts

@@ -29,7 +29,7 @@ async function run() {
           repo: github.context.repo.repo,
           number: github.context.issue.number,
         });
-        await prLinter.validatePullRequestTarget(github.context.payload.sha);
+        await prLinter.validatePullRequestTarget(github.context.sha);
     }
   } catch (error: any) {
     core.setFailed(error.message);

tools/@aws-cdk/prlint/lint.ts

@@ -453,7 +453,7 @@ export class PullRequestLinter {
         await this.assessNeedsReview(pr);
       }
     } catch (e) {
-      console.log('assessing review failed: ', e);
+      console.log(`assessing review failed for sha ${sha}: `, e);
     }
   }