feat: add the ability to declare safe tools in a cross-build environment. (#2317)

* Add the ability to declare safe tools in a cross-build environment.

* Add an xfail if cmake isn't available on the test machine.

* Placate linter regarding positional args.

* Rework test to provide more robust confirmation of safe tools.

* Remove a test skip condition that is no longer needed.

Co-authored-by: Joe Rickerby <joerick@mac.com>

* Rename the setting to xbuild-tools.

* Add docs to clarify that xbuild-tools is transitive.

* Raise a warning if xbuild-tools isn't defined.

* Correct a bad copy-paste in the schema generator.

* .. and now fix the indentation.

* Move sentinel handling earlier into the parsing process.

* Remove serialization from tests that won't start a test suite.

---------

Co-authored-by: Joe Rickerby <joerick@mac.com>

bin/generate_schema.py

@@ -181,9 +181,12 @@ properties:
   musllinux-x86_64-image:
     type: string
     description: Specify alternative manylinux / musllinux container images
-  repair-wheel-command:
+  xbuild-tools:
+    description: Binaries on the path that should be included in an isolated cross-build environment
     type: string_array
+  repair-wheel-command:
     description: Execute a shell command to repair each built wheel.
+    type: string_array
   skip:
     description: Choose the Python versions to skip.
     type: string_array
@@ -273,6 +276,7 @@ items:
       properties:
         before-all: {"$ref": "#/$defs/inherit"}
         before-build: {"$ref": "#/$defs/inherit"}
+        xbuild-tools: {"$ref": "#/$defs/inherit"}
         before-test: {"$ref": "#/$defs/inherit"}
         config-settings: {"$ref": "#/$defs/inherit"}
         container-engine: {"$ref": "#/$defs/inherit"}