[bazel] Use WPILIB_VERSION automatically for version template generation (#8117)

shared/bazel/rules/gen/gen-version-file.bzl

@@ -1,9 +1,10 @@
 def _generate_version_file_impl(ctx):
     out = ctx.actions.declare_file(ctx.attr.output_file)
+    wpilib_version = ctx.var.get("WPILIB_VERSION")
     ctx.actions.expand_template(
         output = out,
         template = ctx.file.template,
-        substitutions = {"${wpilib_version}": "TODO - Built with bazel"},
+        substitutions = {"${wpilib_version}": wpilib_version},
     )
     return [DefaultInfo(files = depset([out]))]