use 'rapids-init-pip' in wheel CI, other CI changes

[jameslamb]

Description

Proposes a batch of small, mostly-unrelated changes to building and packaging.

These are all part of RAPIDS-wide efforts to make it easier to reproduce CI locally and to use artifacts from one project's CI in another project's CI.

Contributes to rapidsai/shared-workflows#356

• explicitly provides an input for script to workflows using it, instead of relying on a default value coming from the workflow file

Contributes to rapidsai/shared-workflows#337

• switches from input run_script to script in uses of the custom-job workflow

Contributes to rapidsai/build-planning#179

• adds a call to rapids-init-pip near the beginning of all CI scripts that use pip
• removes redefinitions of PIP_CONSTRAINT in scripts, in favor of using the one set up by rapids-pip-init

Contributes to rapidsai/gha-tools#145

• removes unnecessary uses of rapids-configure-conda-channels in builds scripts using rattler-build

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[jameslamb]

/ok to test

[bdice]

/merge