allow manylinux compatibility override via _manylinux module. (#6039)

## Summary
resolves https://github.com/astral-sh/uv/issues/5915, not entirely sure
if `manylinux_compatible` should be a separate field in the JSON
returned by the interpreter or there's some way to use the existing
`platform` for it.

## Test Plan
ran the below
```
rm -rf .venv
target/debug/uv venv
# commenting out the line below triggers the change..
# target/debug/uv pip install no-manylinux
target/debug/uv pip install cryptography --no-cache
```

is there an easy way to add this into the existing snapshot-based test
suite? looking around to see if there's a way that doesn't involve
something implementation-dependent like mocks.

~update: i think the output does differ between these two, so probably
we can use that.~ i lied - that "building..." output seems to be
discarded.

crates/uv-python/src/lib.rs

@@ -210,6 +210,7 @@ mod tests {
                             },
                             "arch": "x86_64"
                         },
+                        "manylinux_compatible": true,
                         "markers": {
                             "implementation_name": "{IMPLEMENTATION}",
                             "implementation_version": "{FULL_VERSION}",