Fix --path flag when using wolfictl update

[willdollman]

When the --path flag is used with wolfictl update, the user-provided PkgPath is not used when referencing the melange config file path in pkg/update/update.go.

This results in the update operations assuming that the config file that's being updated is in the root of the repository, rather than under the user-provided PkgPath.

I've set up a minimal repo to reproduce the issue, where melange config files sit in the wolfi-packages/ directory:

$ wolfictl update --dry-run --create-issues=false --path='wolfi-packages/' https://github.com/willdollman/melange-config-example

[...]

# wolfictl tries to update ./qdrant.yaml rather than ./wolfi-packages/qdrant.yaml

2024/04/17 11:08:41 INFO 2024/04/17 11:08:41 wolfictl update: qdrant: failed to bump package qdrant to version 1.8.4: open qdrant.yaml: no such file or directory

With the changes in this PR:

go run main.go update --create-issues=false --path='wolfi-packages/' https://github.com/willdollman/melange-config-example

[...]
# Update succeeds
2024/04/17 12:12:46 INFO 2024/04/17 12:12:46 wolfictl update: after bump: qdrant git status: On branch wolfictl-95b72f3f-a033-425a-8228-3f23073d07d6
[...]