tests: cache managed Python installs

- flip `with_managed_python_dirs()` to set `UV_PYTHON_CACHE_DIR`
- add `without_python_download_cache()` for the cold-cache scenarios
- keep the offline test pointed at a fresh temp cache so it still fails the right way

Author: nooscraft

crates/uv/tests/it/common/mod.rs

@@ -487,16 +487,26 @@ impl TestContext {
     /// Use a shared global cache for Python downloads.
     #[must_use]
     pub fn with_python_download_cache(mut self) -> Self {
-        self.extra_env.push((
-            EnvVars::UV_PYTHON_CACHE_DIR.into(),
-            // Respect `UV_PYTHON_CACHE_DIR` if set, or use the default cache directory
-            env::var_os(EnvVars::UV_PYTHON_CACHE_DIR).unwrap_or_else(|| {
-                uv_cache::Cache::from_settings(false, None)
-                    .unwrap()
-                    .bucket(CacheBucket::Python)
-                    .into()
-            }),
-        ));
+        let key: OsString = EnvVars::UV_PYTHON_CACHE_DIR.into();
+        self.extra_env.retain(|(existing, _)| existing != &key);
+        let value = env::var_os(EnvVars::UV_PYTHON_CACHE_DIR).unwrap_or_else(|| {
+            uv_cache::Cache::from_settings(false, None)
+                .unwrap()
+                .bucket(CacheBucket::Python)
+                .into()
+        });
+        self.extra_env.push((key, value));
+        self
+    }
+
+    /// Disable the shared global cache for Python downloads.
+    #[must_use]
+    pub fn without_python_download_cache(mut self) -> Self {
+        let key: OsString = EnvVars::UV_PYTHON_CACHE_DIR.into();
+        self.extra_env.retain(|(existing, _)| existing != &key);
+        let empty_cache = self.temp_dir.child("python-cache-empty");
+        self.extra_env
+            .push((key, empty_cache.as_os_str().to_owned()));
         self
     }
 
@@ -514,6 +524,8 @@ impl TestContext {
     pub fn with_managed_python_dirs(mut self) -> Self {
         let managed = self.temp_dir.join("managed");
 
+        self = self.with_python_download_cache();
+
         self.extra_env.push((
             EnvVars::UV_PYTHON_BIN_DIR.into(),
             self.bin_dir.as_os_str().to_owned(),

crates/uv/tests/it/python_install.rs

@@ -2679,18 +2679,19 @@ fn python_install_no_cache() {
         "cpython-3.12.[PATCH]-[DATE]-[PLATFORM].tar.gz",
     ));
     filters.push((r"releases/download/\d{8}/", "releases/download/[DATE]/"));
+    let offline_cache = context.temp_dir.child("offline-python-cache");
     uv_snapshot!(filters, context
         .python_install()
         .arg("3.12")
-        .arg("--offline"), @r"
+        .arg("--offline")
+        .env(EnvVars::UV_PYTHON_CACHE_DIR, offline_cache.as_os_str()), @r"
     success: false
     exit_code: 1
     ----- stdout -----
 
     ----- stderr -----
     error: Failed to install cpython-3.12.12-[PLATFORM]
-      Caused by: Failed to download https://github.com/astral-sh/python-build-standalone/releases/download/[DATE]/cpython-3.12.[PATCH]-[DATE]-[PLATFORM].tar.gz
-      Caused by: Network connectivity is disabled, but the requested data wasn't found in the cache for: `https://github.com/astral-sh/python-build-standalone/releases/download/[DATE]/cpython-3.12.[PATCH]-[DATE]-[PLATFORM].tar.gz`
+      Caused by: An offline Python installation was requested, but cpython-3.12.[PATCH]-[DATE]-[PLATFORM].tar.gz) is missing in offline-python-cache
     ");
 }
 
@@ -2709,9 +2710,10 @@ fn python_install_emulated_macos() {
     if !arch_status.is_ok_and(|x| x.success()) {
         // Rosetta is not available to run the x86_64 interpreter
         // fail the test in CI, otherwise skip it
-        if env::var("CI").is_ok() {
-            panic!("x86_64 emulation is not available on this CI runner");
-        }
+        assert!(
+            env::var("CI").is_err(),
+            "x86_64 emulation is not available on this CI runner"
+        );
         debug!("Skipping test because x86_64 emulation is not available");
         return;
     }