fix(pkgbuild.sh): Use grep -Fx to check for pkgrel and pkgver declaration

Based on @gromit's comment, I created this MR which uses grep -Fx to keep the pkgver and pkgrel as literals. -x is necessary since anchors are not possible in fixed strings and the string must match the whole line.

What do you think?

Fixes: #242

src/lib/util/pkgbuild.sh

@@ -26,7 +26,7 @@ pkgbuild_set_pkgver() {
 		warning 'setting pkgver variable has no effect if the PKGBUILD has a pkgver() function'
 	fi
 
-	if ! grep --extended-regexp --quiet --max-count=1 "^pkgver=${pkgver}$" PKGBUILD; then
+	if ! grep -Fx --quiet --max-count=1 "pkgver=${pkgver}" PKGBUILD; then
 		die "Non-standard pkgver declaration"
 	fi
 	sed --regexp-extended "s|^(pkgver=)${pkgver}$|\1${new_pkgver}|g" --in-place PKGBUILD
@@ -38,7 +38,7 @@ pkgbuild_set_pkgrel() {
 	local new_pkgrel=$1
 	local pkgrel=${pkgrel}
 
-	if ! grep --extended-regexp --quiet --max-count=1 "^pkgrel=${pkgrel}$" PKGBUILD; then
+	if ! grep -Fx --quiet --max-count=1 "pkgrel=${pkgrel}" PKGBUILD; then
 		die "Non-standard pkgrel declaration"
 	fi
 	sed --regexp-extended "s|^(pkgrel=)${pkgrel}$|\1${new_pkgrel}|g" --in-place PKGBUILD