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.

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