fix(ci): Fix codecov_src_go by specifying go version

[voisey]

Prerequisite for #12151.

Summary

codecov_src_go is failing here, for example, because os.ReadFile and os.WriteFile are undefined. I believe this is because an inconsistent version of Go is being used for the test (1.15.15), since a version >= 1.16 is required to use these functions. Before 1.16, these functions were instead defined by the io/ioutil package (see e.g. here). Here, I specify version 1.18, which is the latest available release, which requires updating actions/setup-go from v2 to v3.

Test Plan

Checked that codecov_src_go downloads Go 1.18 as desired and passes: https://github.com/magma/magma/runs/5717337059?check_suite_focus=true.

Additional Information

• This change is backwards-breaking

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[uri200]

will codecov still work with current go version without this #12151?