Fix unit tests on Android (#10224)

# Description

* The path to the binaries for tests is slightly incorrect. It is
missing the build target when it is set with the `CARGO_BUILD_TARGET`
environment variable. For example, when `CARGO_BUILD_TARGET` is set to
`aarch64-linux-android`, the path to the `nu` binary is:

  `./target/aarch64-linux-android/debug/nu`

  rather than

  `./target/debug/nu`

This is common on Termux since the default target that rustc detects can
cause problems on some projects, such as [python's `cryptography`
package](https://github.com/pyca/cryptography/issues/7248).
  
This technically isn't a problem specific to Android, but is more likely
to happen on Android due to the latter.
* Additionally, the existing variable named `NUSHELL_CARGO_TARGET` is in
fact the profile, not the build target, so this was renamed to
`NUSHELL_CARGO_PROFILE`. This change is included because without the
rename, the build system would be using `CARGO_BUILD_TARGET` for the
build target and `NUSHELL_CARGO_TARGET` for the build profile, which is
confusing.
* `std path add` tests were missing `android` test

# User-Facing Changes

For those who would like to build nushell on Termux, the unit tests will
pass now.

crates/nu-std/tests/test_std.nu

@@ -27,7 +27,13 @@ def path_add [] {
         assert equal (get_path) ["fooooo", "foo", "bar", "baz"]
 
         load-env {$path_name: []}
-        let target_paths = {linux: "foo", windows: "bar", macos: "baz"}
+
+        let target_paths = {
+            linux: "foo",
+            windows: "bar",
+            macos: "baz",
+            android: "quux",
+        }
 
         std path add $target_paths
         assert equal (get_path) [($target_paths | get $nu.os-info.name)]