build.plat: fix toolchain environment variable check, #2.

Fixes typo introduced in commit 78b90fb.

Author: whitequark

amaranth/build/plat.py

@@ -89,7 +89,7 @@ def build(self, elaboratable, name="top",
         #     may fail.
         # This is OK because even if `require_tool` succeeds, the toolchain might be broken anyway.
         # The check only serves to catch common errors earlier.
-        if do_build and not self._toolchain_env_var not in os.environ:
+        if do_build and self._toolchain_env_var not in os.environ:
             for tool in self.required_tools:
                 require_tool(tool)