archbuild: use better umask

In commit 40a90e2cab we tried to protect
against system umasks resulting in unreadable chroots. However, we tried
to do this in a targeted manner due to not wanting to fiddle with
permissions for user-owned files. Unfortuantely, mkdir -p -m755 does not
actually work that way -- the parent directory is created with broken
permissions. We need umask.

Run umask and mkdir in a subshell to prevent leakage.

Signed-off-by: Eli Schwartz <eschwartz@archlinux.org>

archbuild.in

@@ -68,7 +68,7 @@ if ${clean_first} || [[ ! -d "${chroots}/${repo}-${arch}" ]]; then
 	lock_close 9
 
 	rm -rf --one-file-system "${chroots}/${repo}-${arch}"
-	mkdir -m755 -p "${chroots}/${repo}-${arch}"
+	(umask 0022; mkdir -p "${chroots}/${repo}-${arch}")
 	setarch "${arch}" mkarchroot \
 		-C "${pacman_config}" \
 		-M "${makepkg_config}" \