Fix RTD builds by restricting test-nightly to linux/macOS platforms (#11021)

The `test-nightly` environment uses pandas nightly wheels from PyPI,
which currently don't have win-64 builds available. This causes
`pixi lock` to fail when solving for all platforms.

RTD builds fail because they have no lock file cache (unlike GitHub
Actions CI which caches pixi.lock). When RTD runs `pixi install -e doc`,
pixi must generate the lock file from scratch, which fails on the
unsolvable test-nightly/win-64 combination.

This restriction can be removed once pandas nightly provides win-64
wheels again.

Co-authored-by: Claude <noreply@anthropic.com>

Author: max-sixty

pixi.toml

@@ -143,6 +143,12 @@ sparse = "0.15.*"
 toolz = "0.12.*"
 zarr = "2.18.*"
 
+# TODO: Remove `platforms` restriction once pandas nightly has win-64 wheels again.
+# Without this, `pixi lock` fails because it can't solve the nightly feature for win-64,
+# which breaks RTD builds (RTD has no lock file cache, unlike GitHub Actions CI).
+[feature.nightly]
+platforms = ["linux-64", "osx-arm64"]
+
 [feature.nightly.dependencies]
 python = "*"