fix: bootstrapping script to not use multiline f-strings

[jacky8hyf]

With bootstrap_impl=system_python, after
#2607, host Python 3.6 and above is required because of the use of multiline f-strings. For maximum backwards compatibility (because this is a host dependency), do not use multiline f-strings.

This issue naturally does not apply to bootstrap_impl=script because there are no host Python dependency.

Link: #2607

[rickeylev]

Thanks for the PR!

This is a simple change, so LGTM.

Note that there's no guarantees future changes won't introduce incompatibility with old python versions. We won't actively guard against that, but PRs to fix it will likely accepted.