PYTHON-6084 Rework Evergreen Python interpreter selection for uv

[blink1073]

Rework how Evergreen selects the Python interpreter for uv tasks. Task config now sets UV_PYTHON directly (a version like 3.10/3.14t, an implementation like pypy3.11, or an absolute path) instead of a TOOLCHAIN_VERSION expansion that a script translated per host, so the interpreter lookup happens on the host.

• Task config sets UV_PYTHON directly; removed TOOLCHAIN_VERSION and regenerated the Evergreen task and variant configs.
• setup-uv-python.sh resolves the interpreter on the host, setting UV_PYTHON_SEARCH_PATH/UV_PYTHON_PREFERENCE against a matching toolchain Python, and defaults to CPython 3.10 when a task does not pin one.
• setup_tests.py/utils.py/run-tests.sh propagate and scrub the UV_PYTHON search-path vars; the perf task clears them so its exact-patch interpreter resolves normally.

PYTHON-6084

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