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

added 5 commits

• 0ed76257...b88dec32 - 4 commits from branch archlinux:master
• 2745f072 - fix(pkgbuild.sh): Use grep -Fx to check for pkgrel and pkgver declaration

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added 36 commits

• 2745f072...4926d9d8 - 35 commits from branch archlinux:master
• b66d2a49 - fix(pkgbuild.sh): Use grep -Fx to check for pkgrel and pkgver declaration

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