env_setup: Don't write the spinner to files

Avoid writing backspace character to logs files as this just makes the
log harder to read.

No-Docs-Update-Reason: no API change
Change-Id: I4040e2198a610fe13d655ab37b95c73c92f50c02
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/48400
Reviewed-by: Rob Mohr <mohrr@google.com>
Commit-Queue: Michael Spang <spang@google.com>