Incorrect plurals in string "%d source files given on command line"

Describe the bug

https://github.com/sphinx-doc/sphinx/blob/76110c3ea05e4eef02bc1689c4b9253aca743392/sphinx/builders/__init__.py#L298-L299

The above code assumes the placeholder %d is plural, but it can be singular. This causes a weird rendered sentence of "1 source files...".

Besides, different languages have different plural rules, so the best would be to make this a ngettext / plural-form string so that the translator can translate correctly for number of plurals their language has.

How to Reproduce

1. Run: sphinx-build -M html . _build any_source_file
2. See an output like:

...
building [html]: 1 source files given on command line
...

Environment Information

Platform:              linux; (Linux-6.10.10-arch1-1-x86_64-with-glibc2.40)
Python version:        3.12.6 (main, Sep  8 2024, 13:18:56) [GCC 14.2.1 20240805])
Python implementation: CPython
Sphinx version:        8.0.2
Docutils version:      0.21.2
Jinja2 version:        3.1.4
Pygments version:      2.18.0

Sphinx extensions

No response

Additional context

No response

Start at sphinx/builders/init.py lines 298-299 and reproduce the output with sphinx-build -M html . _build and a single source file. Check how the message is translated and represented in the project's tests or localization workflow. Done means the singular output is grammatically correct and the message supports language-specific plural forms.