Support ?= in [environment] section

In [this PR](https://github.com/zeek/zeek/pull/3823) I've used the following pattern to allow users to override an environment variable externally:
```
[environment]
HILTI_JIT_PARALLELISM=`bash -c 'echo ${HILTI_JIT_PARALLELISM:-1}'`
```

Would it make sense to support `HILTI_JIT_PARALLELISM?=1` similar to the Makefile style? For "tweakable" environment variables?