build: debug builds shouldn't be stripped

[ee7]

Scenario

Run nimble build -d:debug

Expected behavior

It produces a binary with debug info.

Observed behavior

It produces a binary without debug info.

Problem

We make nimble build produce a release build by default, requiring the user to pass -d:debug for a debug build:

https://github.com/crashappsec/chalk/blob/ca217bd77211a0a6125840fd9167e3f76340738b/config.nims#L9-L13

and in the nimble file we intend to strip only the release binary:

https://github.com/crashappsec/chalk/blob/ca217bd77211a0a6125840fd9167e3f76340738b/chalk.nimble#L78-L81

However the above doesn't work, because nimble doesn't support checking custom defines in the nimble file.