fix(nodeenv): detect musl regardless of the host triplet vendor

[ekalinin]

Closes #290.

Problem

is_x86_64_musl() compared sysconfig.get_config_var('HOST_GNU_TYPE') to the literal x86_64-pc-linux-musl. That vendor field is not fixed - it is whatever the person who built CPython passed to ./configure. Alpine builds its own python3 package with --build=x86_64-alpine-linux-musl, so the check returned False on every Alpine release I tried:

image	python	HOST_GNU_TYPE
alpine:3.14	3.9.17	x86_64-alpine-linux-musl
alpine:3.19	3.11.14	x86_64-alpine-linux-musl
alpine:3.22	3.12.14	x86_64-alpine-linux-musl
alpine:edge	3.14.7	x86_64-alpine-linux-musl
python:3.13-alpine	3.13.15	x86_64-pc-linux-musl

With detection off, all three of its call sites go the wrong way: the archive postfix (get_node_bin_url), the release filter (_has_platform_build) and the switch to unofficial-builds.nodejs.org (main). nodeenv reports a successful install and the resulting node fails with not found, because the glibc build needs /lib64/ld-linux-x86-64.so.2.

The official python:*-alpine images build CPython from source and get the default pc vendor, which is why CI never caught this - only the distro's own apk add python3 is affected, and that is exactly what a minimal Alpine container running pre-commit uses.

Fix

Match the arch and the -linux-musl suffix, ignore the vendor field. or '' keeps the call safe where HOST_GNU_TYPE is undefined (Windows), which the old == comparison handled implicitly.

test_mirror_option() kept its own two-entry list of triplet spellings - it now reuses the helper instead, so the suite stops duplicating the logic it is testing around.

Verification

• alpine:3.22 with the distro python3, --platform linux/amd64: install succeeds and bin/node --version prints v22.14.0. Before the change the same run died on the missing ld-linux-x86-64.so.2.
• debian:12-slim: unchanged, still takes the glibc build from nodejs.org.
• flake8 --extend-ignore=E127 nodeenv.py tests setup.py clean, pytest -m 'not integration' tests green.