... and switch our default compilation mode to `dbg`.

Under `fastbuild`, built binaries are stripped. This is not the case
with `dbg`. Have `dev doctor` recommend using `dbg`, and either way
make `dev` resilient to whatever kind of `--compilation_mode` you're
using.

In principle or theory `dbg` is slower than `fastbuild`, but for the Go
compiler it generates debuggable binaries by default and you have to
opt-in to stripping, so it shoould make no real difference.

Now, we think of the `--compilation_mode` simply as follows: `dbg` is
the default that everyone is opted into by default unless they
explicitly set `-c opt` (we use this for release). For now I don't see
a reason anyone would need `fastbuild`.

Epic: CRDB-17171
Release note: None
Closes: cockroachdb#106820

We added support for building in non-default configurations/compilation
modes in a previous PR, but this was buggy and we were never actually
using the configuration arguments in `bazel info`, so staging was always
happening from the `fastbuild` location. This PR fixes that buggy
behavior, so staging will happen from the appropriate source location.

Epic: CRDB-17171
Release note: None