fix(integ-runner): update workflow error message is inaccurate (#27924)

The previously printed command doesn't actual work.

----

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

packages/@aws-cdk/integ-runner/README.md

@@ -16,7 +16,6 @@
 
 <!--END STABILITY BANNER-->
 
-
 ## Overview
 
 This tool has been created to be used initially by this repo (aws/aws-cdk). Long term the goal is
@@ -38,12 +37,15 @@ integ-runner [ARGS] [TEST...]
 This will look for all files that match the naming convention of `/integ.*.js$/`. Each of these files will be expected
 to be a self contained CDK app. The runner will execute the following for each file (app):
 
-1. Check if a snapshot file exists (i.e. `/*.snapshot$/`)
-2. If the snapshot does not exist
-	2a. Synth the integ app which will produce the `integ.json` file
+1. Check if snapshot files exist (i.e. `*.snapshot/**`)
+2. If the snapshot does not exist\
+   a) Synth the integ app which will produce the `integ.json` file
 3. Read the `integ.json` file which contains instructions on what the runner should do.
 4. Execute instructions
 
+All snapshot files (i.e. `*.snapshot/**`) must be checked-in to version control.
+If not, changes cannot be compared across systems.
+
 ### Options
 
 - `--update-on-failed` (default=`false`)

packages/@aws-cdk/integ-runner/lib/runner/integ-test-runner.ts

@@ -141,8 +141,9 @@ export class IntegTestRunner extends IntegRunner {
         });
       } catch (e) {
         logger.warning('%s\n%s',
-          `Could not checkout snapshot directory ${this.snapshotDir} using these commands: `,
-          `git merge-base HEAD ${baseBranch} && git checkout {merge-base} -- ${relativeSnapshotDir}`,
+          `Could not checkout snapshot directory '${this.snapshotDir}'. Please verify the following command completes correctly:`,
+          `git checkout $(git merge-base HEAD ${baseBranch}) -- ${relativeSnapshotDir}`,
+          '',
         );
         logger.warning('error: %s', e);
       }