No output that meets expectations.

````
contenthub-1 on  dev is 📦 v0.14.1-dev.43 via ⬢ v22.15.0
➜ envsubst --version
envsubst (GNU gettext-runtime) 0.25
Copyright (C) 2003-2025 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
Written by Bruno Haible.

contenthub-1 on  dev is 📦 v0.14.1-dev.43 via ⬢ v22.15.0
➜ echo 'welcome ${USER:=312}' | envsubst
welcome ${USER:=312}
````

The issue shows that envsubst is not expanding the default value syntax ${USER:=312}. Look at the Go source code for parsing and substitution logic, likely in the main package or a substitution function. Run the failing command from the issue to reproduce, then check how the tool handles default value assignments. Verify the fix by testing with the provided echo command.

索引モデルが issue の本文から書いたものです。