Allow setting build envs externally

[jprochazk]

Previously:

export CI=1
export IS_IN_RERUN_WORKSPACE=1

cargo build  # runs `git` commands

We need to be able to specify some of the build env vars externally. This is now possible:

export CI=1
export IS_IN_RERUN_WORKSPACE=1
export GIT_HASH=$(git rev-parse HEAD)
export GIT_BRANCH=$(git symbolic-ref --short HEAD)
export DATETIME=$(date -u +"%Y-%m-%dT%H:%M:%SZ")

cargo build  # does NOT run `git` commands

[github-actions]

Web viewer built successfully.

Result	Commit	Link	Manifest
✅	9c3bf15	https://rerun.io/viewer/pr/12164	+nightly +main

View image diff on kitdiff.

^{Note: This comment is updated whenever you push a commit.}