This change is mainly motivated by an issue with the environment
printing I added in PR 82403: multiple rustc invocations progress
in parallel, and the environment output, spanning multiple lines,
gets interleaved in ways make it difficult to extra the enviroment settings.

(This aforementioned difficulty is more of a hiccup than an outright
show-stopper, because the environment variables tend to be the same for all of
the rustc invocations, so it doesn't matter too much if one mixes up which lines
one is looking at. But still: Better to fix it.)

…-SHIM]` unconditionally.

1. I added `--test` based on review feedback from simulacrum: I decided I would
rather include such extra context than get confused later on by its absence.
(However, I chose to encode it differently than how `[RUSTC-TIMING]` does... I
don't have much basis for doing so, other than `--test` to me more directly
reflects what it came from.)

2. I also decided to include `[RUSTC-SHIM]` at start of all of these lines
driven by the verbosity level, to make to clear where these lines of text
originate from. (Basically, I skimmed over the output and realized that a casual
observer might not be able to tell where this huge set of new lines were coming
from.)