pkgctl: inconsistent argument specification

@maximbaz noted in their feedback that currently it is not very consistent how you specify arguments with input:

$ pkgctl build --help
BUILD OPTIONS
    --arch ARCH          Specify architectures to build for (disables auto-detection)
    --repo REPO          Specify a target repository (disables auto-detection)
    -I, --install FILE   Install a package into the working copy of the chroot

PKGBUILD OPTIONS
    --pkgver=PKGVER      Set pkgver, reset pkgrel and update checksums
    --pkgrel=PKGREL      Set pkgrel to a given value

I think we should decide for one and roll that out.

Also the provided example does not work because of that mixed behavior:

$ pkgctl build --pkgver 1.42 --release --db-update
==> ERROR: invalid argument: --pkgver

$ pkgctl build --pkgver=1.42 --release --db-update                                                                                                                      ==> Updating pacman database cache
:: Synchronizing package databases...
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