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/mrc.sh

@@ -4,7 +4,7 @@
 # Modifications in this version are Copyright 2021,2023-2025 Leah Rowe.
 # Original copyright detailed in repo: https://review.coreboot.org/coreboot/
 
-eval "`setvars "" MRC_url MRC_url_bkup MRC_hash MRC_board SHELLBALL`"
+eval "$(setvars "" MRC_url MRC_url_bkup MRC_hash MRC_board SHELLBALL)"
 
 extract_refcode()
 {