Apply goimports before checking make sizegen

[frouioui]

Description

After having issues with check_make_sizegen due to a difference between the source file (on which goimports was applied) and the file generated by Jennifer in the import statement, CI builds started failing in this pull request with the following error:

--- FAIL: TestFullGeneration (1.66s)
    sizegen_test.go:31: 
        	Error Trace:	sizegen_test.go:31
        	Error:      	Should be empty, but was ['/home/runner/work/vitess/vitess/go/tools/sizegen/integration/cached_size.go' has changed]
        	Test:       	TestFullGeneration

To alleviate this issue, this pull request changes how the TestFullGeneration test compares the local and generated files. After Jennifer has generated the file, we write its content to a temporary file, execute goimports on it, and finally, we run the comparison between the local and generated files. This ensures that we compare local files against goimports'ed files. And to make sure our local files are always goimports'ed, this pull request adds a new workflow (check_imports) that fails if a file is not goimports'ed.

Adding this new workflow requires the installation of goimports, to do that, it was necessary to upgrade setup-go action from v1 to v2 as explained in this issue: actions/setup-go#27.

Related Issue(s)

Checklist

• Tests were added or are not required
• Documentation was added or is not required

[GuptaManan100]

Everything else LGTM! 🍰