Remove dep pkg manager and excess CI steps

Description

Thanks for bringing this issue up @anuraaga ! I looked through old PRs like #90 and read your description to come up with this PR to remove dep.

~One concern I have was that you said we could~

~> for example #265 becomes trivial to solve by compressing our build down to a single command.~

~so I looked at how we ended up fixing #265 in #267 and noticed these lines:~

go version
echo "GOPATH=${{ github.workspace }}" >> $GITHUB_ENV
echo "${{ github.workspace }}/bin" >> $GITHUB_PATH

~I'm not too familiar with go, but does removing dep mean we don't need these lines anymore?~

~Actually I just tested this on my own branch on my fork and it works fine so I will remove these lines. Please let me know if I'm wrong!~

~Also when you say~

by compressing our build down to a single command.

~Did I do that correctly in this PR because we only have go help download now?~

~If it would be more convenient to link me to dep documentation where I can verify this is not needed please feel free to do that!~

Did a final test with all your comments and the test still passes on my fork. This change should drop the size of the CI a lot!

Fixes #266

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