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bazel: support using SOURCE_DATE_EPOCH to override date
**What this PR does / why we need it**: ports #48708 to apply to the bazel build as well.
Note that bazel already sets `BUILD_TIMESTAMP` using `SOURCE_DATE_EPOCH`, but we have our own timestamp variable, so we need to fix it too.
We could combine this with something like
```shell
SOURCE_DATE_EPOCH=$(git log -1 --format=%cd --date=format:%s)
```
to get closer to repeatable builds. (This uses the timestamp from the last git commit.)
**Release note**:
```release-note
NONE
```
/assign @BenTheElder @mikedanese
05944b1d2c