Update pixi.toml files to use [workspace] instead of deprecated [project]

Pixi deprecated the [project] table in pixi.toml in favour of [workspace].
Updates cowsay-multi-env.toml and the PIXI_TOML_STUB in test_content.py.
pyproject.toml files are left unchanged since [project] is the correct
PEP 517 standard header there.

See also apposed/appose-java@0c26155

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: ctrueden

tests/builder/test_content.py

@@ -36,7 +36,7 @@
 # scheme detection, but deliberately incompatible with the builder under test.
 
 PIXI_TOML_STUB = """\
-[project]
+[workspace]
 name = "stub"
 channels = ["conda-forge"]
 platforms = ["linux-64"]

tests/resources/envs/cowsay-multi-env.toml

@@ -1,4 +1,4 @@
-[project]
+[workspace]
 name = "appose-cowsay-multi-env"
 channels = ["conda-forge"]
 platforms = ["linux-64", "osx-64", "osx-arm64", "win-64"]