Revert "Revert "Revert "all: set User-Agent in external HTTP requests…

Created by: bobheadxi

… (#28086)"""

This reverts commit f38bbfa6176f3546f43cf4c2c1f9c85fbc249c46.

I've rotated the token and the build is still broken by this change (initial revert passed on an agent with the old token, revert of revert failed on the new token)

It's likely the user agent set here is causing GHE to redirect us to a login page:

lvl=warn msg="error cloning repo" repo=github.com/sgtest/go-diff err="error cloning repo: repo github.com/sgtest/go-diff not cloneable: exit status 128 (output follows)\n\nfatal: unable to update url base from redirection:\n  asked for: https://<redacted>@ghe.sgdev.org/sgtest/go-diff/info/refs?service=git-upload-pack\n   redirect: https://ghe.sgdev.org/login?return_to=https%3A%2F%2Fghe.sgdev.org%2Fsgtest%2Fgo-diff%2Finfo%2Frefs%3Fservice%3Dgit-upload-pack"

This breaks access to all the test repositories we have hosted on ghe.sgdev.org (which shows up in the tests as github.com repositories despite not actually being github.com repositories for testing purposes, presumably: https://github.com/sourcegraph/sourcegraph/blob/452f894f8f2026bfa46bc97fc509fe2f90949097/dev/gqltest/search_test.go#L34-L46 )