BUILDBOT: Switch iPhone builds back to tar.bz2 packaging.

This is as per a user request as tar.xz can not be decompressed
directly on iOS devices, so required an intermediate desktop device
to install these builds.

Other tar formats such as bz2 can be reportedly decompressed fine
using iFile etc.

config/master.cfg

@@ -567,7 +567,7 @@ p = {
 	"env": scumm_env_iphone,
 	"disttarget": "iphonebundle",
 	"package": [ "ScummVM.app" ],
-	"archive_format": "tar.xz"
+	"archive_format": "tar.bz2"
 }
 
 scumm_platforms_master["iphone"] = p
@@ -590,7 +590,7 @@ p = {
 	"env": scumm_env_iphone_clang,
 	"disttarget": "iphonebundle",
 	"package": [ "ScummVM.app" ],
-	"archive_format": "tar.xz"
+	"archive_format": "tar.bz2"
 }
 
 scumm_platforms_master["iphone-clang"] = p