Improve test behavior with custom installation directory (#10048)

## Summary

Testing with `UV_PYTHON_INSTALL_DIR` environment variable has some
problems. This PR fix them.

- `UV_PYTHON_INSTALL_DIR` must be an absolute path.
- Cargo tries to find Python executables from each crates in test. If it
is relative path, cargo searches in different directories for each
tests.
- Skip the test asserting help messages.
- Clap shows the current value of the environment variables. If
`UV_PYTHON_INSTALL_DIR` is set, the test fails.

## Test Plan

<!-- How was it tested? -->

All tests pass with
`UV_PYTHON_INSTALL_DIR=/path/to/my/home/uv/target/testpython`.

CONTRIBUTING.md

@@ -47,7 +47,7 @@ Testing uv requires multiple specific Python versions; they can be installed wit
 cargo run python install
 ```
 
-The storage directory can be configured with `UV_PYTHON_INSTALL_DIR`.
+The storage directory can be configured with `UV_PYTHON_INSTALL_DIR`. (It must be an absolute path.)
 
 ### Snapshot testing
 

crates/uv/tests/it/help.rs

@@ -1,3 +1,5 @@
+use uv_static::EnvVars;
+
 use crate::common::{uv_snapshot, TestContext};
 
 #[test]
@@ -452,7 +454,7 @@ fn help_subcommand() {
 fn help_subsubcommand() {
     let context = TestContext::new_with_versions(&[]);
 
-    uv_snapshot!(context.filters(), context.help().arg("python").arg("install"), @r###"
+    uv_snapshot!(context.filters(), context.help().env_remove(EnvVars::UV_PYTHON_INSTALL_DIR).arg("python").arg("install"), @r###"
     success: true
     exit_code: 0
     ----- stdout -----