refactor: make releases grep line more robust

Header files are not always consistent in their format.
'Location' can sometimes be capitalized, hence the '-i'

Regex can too easily turn into lines of indecipherable hieroglyphics  but in this case, due to variance as cited above, regex would probably be more robust than grepping for 'location:' (with a colon)  and if the header was reorganised then '-m 1' could end up up failing.

quickget

@@ -828,7 +828,7 @@ function releases_manjaro() {
 
 function releases_mxlinux() {
     # needs header, so not web_pipe:
-    curl -Ils "https://sourceforge.net/projects/mx-linux/files/latest/download" | grep -i -m 1 'location' | cut -d? -f1 | cut -d_ -f1 | cut -d- -f3
+    curl -Ils "https://sourceforge.net/projects/mx-linux/files/latest/download" | grep -i -E ^'location' | cut -d? -f1 | cut -d_ -f1 | cut -d- -f3
 }
 
 function editions_mxlinux() {