build-using-self: Do not set unnecessary env var (#8859)

The SWIFTPM_CUSTOM_BIN_DIR environment variable was set as it was
required by the Integration Tests when they were standalone Package.
Since merging the Integration Tests Package with the SwiftPM package
Tests, setting the environment variable is no longer necessary.

Update the `build-using-self` script to no longer set this environment
variable.

Author: bkhouri

Utilities/build-using-self

@@ -137,9 +137,6 @@ def set_environment(
 ) -> None:
     os.environ["SWIFTCI_IS_SELF_HOSTED"] = "1"
 
-    # Set the SWIFTPM_CUSTOM_BIN_DIR path
-    os.environ["SWIFTPM_CUSTOM_BIN_DIR"] = str(swiftpm_bin_dir)
-
     # Ensure SDKROOT is configure
     if is_on_darwin():
         sdk_root = call_output(shlex.split("xcrun --show-sdk-path --sdk macosx"))