makechrootpkg: make /tmp inside nspawn larger

systemd by default limits the /tmp folder to be 10% of the host memory:
6f2cea06bf/src/shared/mount-util.h (L33)

This is problematic to our builds because many toolchains opt to put
build artifacts in /tmp, and expecting the host memory to be 10 times
larger is not optimal or even realistic sometimes.

This MR attempts to enlarge it to 50% memory as the host machine's
default value of /tmp. This should be a fair compromise between being
overly conservative and taking up too much memory to crash the system.

src/makechrootpkg.in

@@ -24,6 +24,7 @@ clean_first=0
 run_namcap=0
 run_checkpkg=0
 temp_chroot=0
+tmp_opts="nosuid,nodev,size=50%,nr_inodes=2m"
 
 bindmounts_ro=()
 bindmounts_rw=()
@@ -365,6 +366,7 @@ prepare_chroot
 if arch-nspawn "$copydir" \
 	--bind="${PWD//:/\\:}:/startdir" \
 	--bind="${SRCDEST//:/\\:}:/srcdest" \
+	--tmpfs="/tmp:${tmp_opts}" \
 	"${bindmounts_ro[@]}" "${bindmounts_rw[@]}" \
 	/chrootbuild "${makepkg_args[@]}"
 then