[ci] further simplify CI configurations

[jameslamb]

Follow-up to #6458

Proposes the following simplifications to CI configurations and scripts:

• removes CONDA_ENV environment variable and just hard-codes it in scripts
• removes configurations to run CI on PRs targeting any branch matching the pattern release/*
  • this was added back when we were releasing from branches not targeting master, to avoid releasing the breaking changes building up there in anticipation of 4.0... but that's not the state this project is in any more
• consolidates conda-setup code in test.sh
• reduces duplication of image URI in CUDA CI job config
• removes a Python 2.7 try-catch import in .ci/get_workflow_status.py
• stops writing an env file in QEMU_multiarch job, in favor of just using docker run --env

Also increases the valgrind allow-possibly-lost-bytes limit (see #6213 (comment))

[jameslamb]

/gha run r-valgrind

Workflow R valgrind tests has been triggered! 🚀
https://github.com/microsoft/LightGBM/actions/runs/9201526295

Status: failure ❌.

[jameslamb]

/gha run r-valgrind

Workflow R valgrind tests has been triggered! 🚀
https://github.com/microsoft/LightGBM/actions/runs/9249940386

Status: failure ❌.

[jameslamb]

/gha run r-valgrind

Workflow R valgrind tests has been triggered! 🚀
https://github.com/microsoft/LightGBM/actions/runs/9259654005

Status: success ✔️.

[borchero]

LGTM!

[jameslamb]

I pushed a pydistcheck release last night that broke CI here.

ERROR: found the following unrecognized checks passed via '--ignore': compiled-objects-have-debug-symbols,distro-too-large-compressed

(build link)

Just pushed a commit fixing that: f99a01e

It's so minor and noncontroversial, I'm going to merge this if CI passes, without waiting for another review. We can revert it easily if anyone comes back and disagrees with the changes I made.