Drop added PATHs on folder leave (#1098)

Resolves #1078

Previous behavior: Navigating into a folder with a `Cargo.yml` prepends
the target folders to `PATH`. They never get removed. Navigating into
different cargo folders one after the other leads to repeated PATH
extensions.

New behavior: Navigating into a folder with a `Cargo.yml` prepends the
target folders to `PATH`. Leaving a folder with a `Cargo.yml` drops the
target folders from `PATH`.

There is no guarantee that they were added by the script when entering.
The PATH could have had the target entries before entering. However,
when using this hook, it seems reasonable to assume that this prepend
and drop behavior is desired. It also seems reasonable to assume that
build target folders would not be used for normal util operations.

Author: Kissaki

nu-hooks/nu-hooks/rusty-paths/rusty-paths.nu

@@ -13,13 +13,24 @@
 $env.config = ($env.config | upsert hooks.env_change.PWD {
 	append {
 		condition: {|_, after| ($after | path join 'Cargo.lock' | path exists) }
-		code: {
+		code: {|_, after|
 			$env.PATH = (
 				$env.PATH
-					| prepend ($env.PWD | path join 'target' 'debug')
-					| prepend ($env.PWD | path join 'target' 'release')
+					| prepend ($after | path join 'target' 'debug')
+					| prepend ($after | path join 'target' 'release')
 					| uniq
-				)
+			)
+		}
+	}
+  | append {
+		condition: {|before, _| ($before | default '' | path join 'Cargo.lock' | path exists) }
+		code: {|before, _|
+			$env.PATH = (
+				$env.PATH
+					| where $it != ($before | path join 'target' 'debug')
+					| where $it != ($before | path join 'target' 'release')
+					| uniq
+			)
 		}
 	}
 })