CI: build one recipe at a time on every platform, temporarily (#162)

rattler-build 0.77 gives two variants of a recipe started in the same
second the same build directory (prefix-dev/rattler-build#2898). On the
build branches beluga's two hdf5 variants are the first builds of their
job, so humble's and jazzy's Linux and macOS builds fail every time.
Revert once prefix-dev/rattler-build#2899 is released.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Author: Tobias-Fischer

tools/robostack.py

@@ -114,7 +114,11 @@ def run_tee(cmd: list[str], cwd: Path) -> tuple[int, bytes]:
 
 # a build job's recipes side by side, with a shared jobserver keeping them at the runner's
 # cores in total; Windows has no jobserver, and was no faster with two (#154)
-PARALLEL_BUILDS = "1" if sys.platform == "win32" else f"3:{os.cpu_count()}"
+# Temporarily one at a time everywhere: rattler-build 0.77 gives two variants of a recipe
+# started in the same second the same build directory, and they fail ("Path ... already
+# exists", prefix-dev/rattler-build#2898, e.g. beluga's hdf5 variants). Back to
+# f"3:{os.cpu_count()}" off Windows once prefix-dev/rattler-build#2899 is released.
+PARALLEL_BUILDS = "1"
 
 
 def build_ci_parallel(distro: str, recipes: list[str], args: list[str], cwd: Path) -> int: