Specifying sdk-extensions with branch does not work

[ranfdev]

Checklist

• [X] I agree to follow the Code of Conduct that this project adheres to.
• [X] I have searched the issue tracker for a bug that matches the one I want to file, without success.

flatpak-builder version

1.2.2

Flatpak version

1.12.7

How to reproduce

Install an extension with a version suffix:

flatpak install --user org.freedesktop.Sdk.Extension.rust-stable//21.08

Create a manifest with that extension:

"sdk-extensions" : [
    "org.freedesktop.Sdk.Extension.rust-stable//21.08"
]

Run flatpak builder

flatpak-builder --user build manifest.json

Expected Behavior

Given that the required extension is installed, flatpak-builder should find it without issues.

Actual Behavior

error: Requested extension org.freedesktop.Sdk.Extension.rust-stable//21.08 not installed

Additional Information

No response