fix: Remove hardcoded build_args

[allenkallz]

Description of your changes

I ran into a problem when trying to streamline my workflow to build and push artifacts in a single step by setting BUILD_ARGS to --push in ci workflow. Despite this change, build images weren't being pushed to the registry. I later discovered that the BUILD_ARGS environment variable was being overwritten by a hardcoded value within the Makefile during the docker buildx process.

Fixes #
I removed the hardcoded BUILD_ARGS variable

I have:

• Read and followed Crossplane's [contribution process].
• Run make reviewable test to ensure this PR is ready for review.

[jbw976]

I know these args have historically been here for a long time, and they are used in other builds all over the crossplane ecosystem.

How have you gained confidence that removing this won't break the rest of the provider ecosystem?

[allenkallz]

I have tested make build on my local by setting BUILD_ARGS as env variable before and after the change.

I've also tested this change with workflow in my provider-snowflake repository.

here are Github action workflow logs for
Before the change
After the change

For now, the only change affecting local development is that users must now set BUILD_ARGS as an environment variable when running the make build command.

I can enhance this by implementing logic to use a default value for the environment variable if it's not explicitly set, otherwise, it will use the provided environment variable.

[Upbound-CLA]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}