xbmk: don't use backticks for command substitution

the newer way handles escaped characters better, and it
can be nested more easily. it's also more readable.

personally, i prefer the old way, because it's more
minimalist, but it occurs to me that a lot of people
nowadays don't know about backticks, but they do know
of the modern way.

to make the code more readable, i have modernised it.

Signed-off-by: Leah Rowe <leah@libreboot.org>

include/release.sh

@@ -1,7 +1,7 @@
 # SPDX-License-Identifier: GPL-3.0-or-later
 # Copyright (c) 2023-2025 Leah Rowe <leah@libreboot.org>
 
-eval "`setvars "" reldir reldest vdir rsrc relmode`"
+eval "$(setvars "" reldir reldest vdir rsrc relmode)"
 
 release()
 {
@@ -21,7 +21,7 @@ release()
 	[ -e "$reldest" ] && \
 	    err "already exists: \"$reldest\""
 
-	vdir="`mktemp -d || err "can't make vdir"`" || err
+	vdir="$(mktemp -d || err "can't make vdir")" || err
 	vdir="$vdir/$version"
 
 	rsrc="$vdir/${relname}_src"