Support ?= in [environment] section

[awelzel]

In this PR 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?