Make .mill-opts/.mill-jvm-opts env var interpolation require braces, add tests for overriding opts

[lihaoyi]

No description provided.

[lihaoyi]

CC @lefou

[lefou]

The PWD is the client working dir, right?

We seem to interpolate missing env vars to the empty String. Shouldn't we at least print some warning to STDERR?

[lihaoyi]

Yeah PWD is client working dir

Not sure about printing an error; having optional environment variables seems like it would be a common use case, in which case it's not an error for the env var to be missing

[lefou]

Not sure about printing an error; having optional environment variables seems like it would be a common use case, in which case it's not an error for the env var to be missing

I find some kind of defensive programming can avoid lots of hard to spot issues. We could just log it when we run in --debug mode. But I think we don't fully parse the cli in the client. (If we do, we should also print which additional files we read in --debug mode. Can be another PR though.)

[lefou]

If we don't log it, we should document what happens with undefined env vars.

[lefou]

We don't interpolate .mill-vars .mill-opts currently. But I think we should. For consistency, but also because it's probably the natural way to pass env vars to system properties.

[lihaoyi]

@lefou what's .mill-vars?

[lefou]

@lefou what's .mill-vars?

A typo. I meant .mill-opts.