Since commit fb182159 (Ninja: clean ninja metadata once generated,
2019-05-13, v3.17.0-rc1~207^2) we recompact the ninja deps log during
regeneration.  That does not make sense during a build, so skip it if we
are regenerating during a build.

This problem went unnoticed previously because on non-Windows platforms
the deps log is just overwritten again by the outer build.  On Windows
platforms, recompaction during the build fails, but we did not actually
try to do that until commit 11f42593 (Ninja: Clean metadata after
regen during build on Windows with 1.10.2+, 2020-11-30, v3.19.2~29^2~1).

Fixes: #21916