Added option to explicitly state `pkgname`

This action builds an validates Arch Linux package.
The directory containing PKGBUILD and .SRCINFO files can be specified in pkgbuild\_dir if not in the root of the repo (default is '.')

Example Usage

Create .github/workflows/pkgbuild.yml (or other filename) with this content. This exact code will work for most repos

name: pkgbuild

on: push

jobs:
  pkgbuild_job:
    runs-on: ubuntu-latest
    steps:
    - uses: actions/checkout@v2
    - uses: adig-pkgs/arch-pkgbuild-builder@v2.1

This currently doesn't work for split-packages (ie. if your PKGBUILD contains pkgname=('a-aef' 'b-aef' 'c-aef') # my package)

Differences from original repo -

• Uses pkgbuild_dir variable instead of 'pkgname' to be used as directory containing the PKGBUILD
• You don't need to tell the 'pkgname', the script automatically extracts that info from PKGBUILD file
• Everything has defaults, and with point 2 above, you need 0 configuration on most repos

Why this fork ?

1. Because I have PKGBUILD in root of repo.
2. Zero config use (Just add uses: '', and it should work)