Fixes #16940.

Currently, if a network timeout occurs, uv displays the timeout as 30s. This leads users to try setting UV_HTTP_TIMEOUT=60s, which fails because it expects a plain integer.

This PR:

1. Improves the error message when a network timeout occurs to explicitly state that the value should be an integer in seconds and provides an example.
2. Updates the environment variable parsing logic to provide a specific suggestion (expected an integer (in seconds)) when parsing any variable containing TIMEOUT fails and the value ends with s.

Verified with:

UV_HTTP_TIMEOUT=60s cargo run venv
# error: Failed to parse environment variable `UV_HTTP_TIMEOUT` with invalid value `60s`: invalid digit found in string; expected an integer (in seconds), e.g., '60' instead of '60s'