cleanup pure Perl use
This quiets down tests when the optional Inline::C is missing.
We do not currently have a hard dependency on Inline::C; and we
should not leave PERL_INLINE_DIRECTORY set in PublicInbox::Spawn
if Inline fails to build.
Leaving PERL_INLINE_DIRECTORY set by Spawn after it fails (due
to missing Inline::C) would cause downstream failures in Gcf2
builds for the same reason. So we should bail out of the Gcf2
build early if Spawn already failed due to missing Inline::C.
The only time we want to be noisy is if a user explicitly sets
PERL_INLINE_DIRECTORY and Inline::C is missing.
This reverts commit
ad8acf7d6484d0a489499742cadadbd4f890ab53.
ad8acf7d6484d0a4 (Gcf2: Create cache folder if missing, 2022-09-08)