Stop preventing `pacman` from being updated

[dscho]

There is a left-over IgnorePkg = pacman line in /etc/pacman.conf; I noticed this while investigating why the symptom (a failing sync workflow) did that gave rise to git-for-windows#88 was not seen in git-sdk-arm64, and that left-over line explains it.

[dscho]

Hah, looks like I accidentally removed pacman from the IgnorePkg line and therefore pacman was updated. The mistake was here:

-          sed -i 's/^#\? *\(IgnorePkg *=\).*/\1 msys2-runtime/' etc/pacman.conf &&
+          sed -i 's/^\(# *\(IgnorePkg *=\).*\| *\(IgnorePkg =.*\)\)/\2\3 msys2-runtime/' etc/pacman.conf &&

i.e. if a commented-out IgnorePkg line was found, it should have been replaced by an un-commented line that only adds msys2-runtime, but if an already uncommented IgnorePkg line was found, it should have appended msys2-runtime.

Oh well, at least this ticket is addressed.