fix Linux/macOS HDF5_PLUGIN_PATH in miniconda.yml (compression plugin tests silently skip on ubuntu/macos)

[glaziermag]

On Linux and macOS, the Tests step in miniconda.yml exports HDF5_PLUGIN_PATH="${CONDA_PREFIX}/hdf5/lib/plugin/" — a directory that doesn't exist in the micromamba env. conda-forge installs the netcdf-c filter plugins at ${CONDA_PREFIX}/lib/hdf5/plugin (the H5_DEFAULT_PLUGINDIR baked into conda-forge hdf5 — hdf5-feedstock build.sh L88 — which the libnetcdf feedstock uses as its plugin install dir). With the variable pointing at the empty path, has_zstd_filter()/has_bzip2_filter()/has_blosc_filter() return False and test_compression_{zstd,bzip2,blosc}.py skip with "filter not available" on every ubuntu/macos job of Build and Test, while the Windows jobs — whose ${CONDA_PREFIX}\Library\hdf5\lib\plugin path is correct — run them. Latest master run: ubuntu 3.12 109 passed, 6 skipped vs windows 3.12 111 passed, 3 skipped, 1 xpassed.

Inside the workflow's env:

$ ls "${CONDA_PREFIX}/hdf5/lib/plugin/"     # what the workflow exports
ls: cannot access '.../hdf5/lib/plugin/': No such file or directory
$ ls "${CONDA_PREFIX}/lib/hdf5/plugin"      # where the plugins are
lib__nch5blosc.so  lib__nch5bzip2.so  lib__nch5zstd.so  ...

Verified on my fork at current master (ad9d7d8), runs 1 and 2: the unchanged path reproduces the three skips on ubuntu and macos (109 passed, 6 skipped), and with this one-line change they run and pass, matching Windows (111 passed, 3 skipped, 1 xpassed). The Windows half of this export was tuned against CI directory listings in #1450 (8f83e40 "set HDF5_PLUGIN_PATH for tests"), but the Linux/macOS half kept the original guess from 2d6c8d4, and the skips keep the jobs green.

Disclosure: found while auditing CI logs with an AI assistant (Claude); I re-read the workflow history and ran the fork verifications myself before opening this.

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