fix: add ref parameter to checkout action for pull_request_target event

[reubenmiller]

Without specifying the given reference, it meant that the PR checks were always being run against the main branch.

[github-actions]

Robot Results

✅ Passed	❌ Failed	⏭️ Skipped	Total	Pass %	⏱️ Duration
26	0	0	26	100	12.595408s

Passed Tests

Name	⏱️ Duration	Suite
Get IoT Url	0.553 s	Account
Get Account ID	0.335 s	Account
Create certificate key pair	0.297 s	Certificates
Create CSR using AWS CA	0.408 s	Certificates
Upload self-signed certificate	0.367 s	Certificates
Create Certificate and Thing	0.681 s	Composite
Start/stop MQTT logger	0.582 s	Logger
Assert message count	2.651 s	Logger
Assert MQTT message	2.690 s	Logger
Create a new Policy	0.170 s	Policies
Deleting a non-existent Policy should not throw an error	0.081 s	Policies
Deleting a non-existent Policy should throw an error	0.078 s	Policies
Assert that a policy exists	0.168 s	Policies
Fails if policy does not exist	0.074 s	Policies
Create Thing with auto cleanup	0.194 s	Thing
Create Thing with Principal	0.628 s	Thing
Delete Thing	0.240 s	Thing
Delete non-existent thing	0.075 s	Thing
Check if thing exists	0.074 s	Thing
Throws an error if thing exists	2.242 s	Thing
Get Local Command Topic	0.002 s	Topics
Get Cloud Command Topic	0.001 s	Topics
Get Telemetry Topic	0.001 s	Topics
Get Registration Topic	0.001 s	Topics
Get cloud topic for the AWS shadow	0.001 s	Topics
Get local topic for the AWS shadow	0.001 s	Topics