Fix platform config values for v1 recipes

[jsmolic]

The code is currently trying to set platform configuration values by checking config.variant object which is not valid as we are not loading config files through conda-build's mechanism and any configuration value will not get applied but rather fall back to the default config.subdir. Fix this by obtaining the settings from the variant config loaded in the rattler compat code.
While at it, cleanup redundant command check in run_rattler().

Description

Checklist - did you ...

• Add a file to the news directory (using the template) for the next release's release notes?
• Add / update necessary tests?
• Add / update outdated documentation?

[beckermr]

The test suite apparently didn't catch what appears to be a pretty fundamental issue here. I think we should add tests specifically for this error.

[jsmolic]

The test suite apparently didn't catch what appears to be a pretty fundamental issue here. I think we should add tests specifically for this error.

Yes, I'm working on a test for this atm

[beckermr]

Ahh sorry. I thought this was ready for review!

[codspeed-hq]

Merging this PR will not alter performance

✅ 5 untouched benchmarks

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_{Comparing jsmolic:v1-platform-fix (815c8fc) with main (e6334c7)}

[ForgottenProgramme]

LGTM!

[jaimergp]

AFAIK, rattler-build does not read target_platform from CBC files. e.g. see conda-forge/conda-forge-bot#6274

Is VariantConfig generic enough to parse it, or does it ignore it?

[jsmolic]

AFAIK, rattler-build does not read target_platform from CBC files. e.g. see conda-forge/conda-forge-bot#6274

Is VariantConfig generic enough to parse it, or does it ignore it?

Yes, I know that the platform config settings will be ignored there if it's attempted with the CLI as it gives a warning.

VariantConfig on the other hand can parse the settings from the cbc file and then we just read and forward those values to PlatformConfig