Remove redundant prefix in failed PEP 517 builds error message

The error handling logic will add the ERROR: prefix already. Including one in the error message results in two ERROR: prefixes.

    ERROR: ERROR: Failed to build installable wheels for some pyproject.toml based projects (simplewheel, singlemodule)

src/pip/_internal/commands/install.py

@@ -433,7 +433,7 @@ def run(self, options: Values, args: List[str]) -> int:
 
             if build_failures:
                 raise InstallationError(
-                    "ERROR: Failed to build installable wheels for some "
+                    "Failed to build installable wheels for some "
                     "pyproject.toml based projects ({})".format(
                         ", ".join(r.name for r in build_failures)  # type: ignore
                     )