dev: move local loopback cache line from `~/.bazelrc` to `.bazelrc.user`

[rickystewart]

There hasn't been any particular reason we put it in ~/.bazelrc before, except that it "doesn't hurt" to have it in the home directory, where you get to take advantage of it in any Bazel workspace you happen to be using. However, as we want to add opt-in support for remote execution to dev (which is incompatible with using the local loopback cache), it now makes more sense to opt in to this on a workspace-by-workspace basis. That means it properly belongs in .bazelrc.user instead.

Here we update the relevant dev doctor check to look in .bazelrc.user instead, and update the auto-fix to delete the line from ~/.bazelrc if it's there.

Part of: CRDB-34610 Epic: CRDB-34137 Release note: None

[blathers-crl[bot]]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

_{:owl: Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}

[cockroach-teamcity]

This change is 

[rickystewart]

bors r=rail

[craig[bot]]

Build succeeded:

• linux_arm64_build
• macos_arm64_build
• check_generated_code
• docker_image_amd64
• unit_tests
• windows_build
• examples_orms
• local_roachtest
• lint
• linux_amd64_build
• macos_amd64_build
• acceptance
• local_roachtest_fips
• linux_amd64_fips_build