[dev] Remove "::set-env" commands in GitHub Actions for CVE-2020-15228

[ghost]

Merge Checklist

All boxes should be checked before merging the PR (just tick any boxes which don't apply to this PR)

• The toolchain has been rebuilt successfully (or no changes were made to it)
• The toolchain/worker package manifests are up-to-date
• Any updated packages successfully build (or no packages were changed)
• All package sources are available
• cgmanifest files are up-to-date and sorted (./cgmanifest.json, ./toolkit/tools/cgmanifest.json, ./toolkit/scripts/toolchain/cgmanifest.json)
• All source files have up-to-date hashes in the *.signatures.json files
• sudo make go-tidy-all and sudo make go-test-coverage pass
• Documentation has been updated to match any changes to the build system
• Ready to merge

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Summary

Patches CVE-2020-15228 in our GitHub Actions scripts using the "::set-env" command (corresponds to PR against 1.0-dev branch #178)

Change Log

• Address deprecation of "::set-env"" command due to CVE-2020-15228
• Replace deprecated command with "Environment File Syntax"

Does this affect the toolchain?

NO

Links to CVEs

• https://nvd.nist.gov/vuln/detail/CVE-2020-15228

Test Methodology

• Tested push functionality in thcrain/test-workflow-set-env branch, tested PR functionality in thcrain/test-workflow-set-env-fix branch
• Both functioned as expected post-change