Add `UV_NO_SOURCES` as an environment variable (#15883)
## Summary
This is an enhancement that makes the cli flag `--no-sources` an
environment variable - "UV_NO_SOURCES"
Why is this a relevant change?
When working across different environments, in our case remote vs local,
we often have our packages hosted in a artifact registry but when
developing locally we build our packages from github. This results in us
using the uv.tool.sources table quite a bit however this then also
forces us to use `--no-sources` for all our remote work.
This change enables us to set an environment variable once and to never
have to type --no-sources after every uv run command again.
## Test Plan
Expanded on the current --no-sources tests, to test when
UV_NO_SOURCES=true/false the behaviour is the same as the flag.
Additionally ensured that the cli overrides the env variable.
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Co-authored-by: Charlie Marsh <charlie.r.marsh@gmail.com>
