fix(pkgbuild.sh): Use fixed string matching to check for pkgrel/pkgver declaration

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

In addition, the following sed command used --regexp-extended to avoid repeating the pkgver= and pkgrel= prefix. This leads to the same issue like above, so rather repeat the prefix than failing at valid versions.

Fixes: #242 (closed)