Fix relocatable nushell activation script (#17036)

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## Summary

<!-- What's the purpose of the change? What does it do, and why? -->
Nushell activation now computes the venv root dynamically via path self
for relocatable venvs, while non-relocatable venvs still embed a quoted
absolute path

Still keep `activate.csh` (maybe delete is also an option)

close https://github.com/astral-sh/uv/issues/16973

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

crates/uv-virtualenv/src/activator/activate.nu

@@ -68,7 +68,7 @@ export-env {
         }
     }
 
-    let virtual_env = '{{ VIRTUAL_ENV_DIR }}'
+    let virtual_env = {{ VIRTUAL_ENV_DIR }}
     let bin = '{{ BIN_NAME }}'
     let path_name = if (has-env 'Path') { 'Path' } else { 'PATH' }
     let venv_path = ([$virtual_env $bin] | path join)

crates/uv-virtualenv/src/virtualenv.rs

@@ -463,8 +463,15 @@ pub(crate) fn create(
             (true, "activate.fish") => {
                 r#"'"$(dirname -- "$(cd "$(dirname -- "$(status -f)")"; and pwd)")"'"#.to_string()
             }
+            (true, "activate.nu") => r"(path self | path dirname | path dirname)".to_string(),
+            (false, "activate.nu") => {
+                format!(
+                    "'{}'",
+                    escape_posix_for_single_quotes(location.simplified().to_str().unwrap())
+                )
+            }
             // Note:
-            // * relocatable activate scripts appear not to be possible in csh and nu shell
+            // * relocatable activate scripts appear not to be possible in csh.
             // * `activate.ps1` is already relocatable by default.
             _ => escape_posix_for_single_quotes(location.simplified().to_str().unwrap()),
         };

crates/uv/tests/it/venv.rs

@@ -1296,6 +1296,12 @@ fn verify_pyvenv_cfg_relocatable() {
     let activate_fish = scripts.child("activate.fish");
     activate_fish.assert(predicates::path::is_file());
     activate_fish.assert(predicates::str::contains(r#"set -gx VIRTUAL_ENV ''"$(dirname -- "$(cd "$(dirname -- "$(status -f)")"; and pwd)")"''"#));
+
+    let activate_nu = scripts.child("activate.nu");
+    activate_nu.assert(predicates::path::is_file());
+    activate_nu.assert(predicates::str::contains(
+        r"let virtual_env = (path self | path dirname | path dirname)",
+    ));
 }
 
 /// Ensure that a nested virtual environment uses the same `home` directory as the parent.