delEnv now works at CT

[timotheecour]

before PR:

• putEnv works at CT
• delEnv silently noop at CT (makes no sense to make this an exception given that putEnv works at CT)

after PR:

• putEnv works at CT
• delEnv works at CT
• the tests in tests/stdlib/tosenv.nim from [superseded] Remove tracking of environment from osenv.nim #18535 can be run at CT via:

template main = ...
static: main()
main()

except for a few tests which would require #13714 to pass (and except for the ones that rely on threads:on)

I can add a changelog if requested but I consider this more of a bugfix.

[Araq]

Bugfix indeed because nimscript.nim contains:

proc delEnv*(key: string) {.tags: [WriteIOEffect].} =
  ## Deletes the environment variable named `key`.
  builtin