Revert change that prevented install tests from actually installing

Author: plemarquand

Sources/SwiftlyCore/Platform.swift

@@ -454,6 +454,16 @@ extension Platform {
             return cmdAbsolute
         }
 
+        // If we're running inside an xctest then we don't have a location for this swiftly.
+        guard let cmdAbsolute,
+              !(
+              (cmdAbsolute.string.hasSuffix("xctest") || cmdAbsolute.string.hasSuffix("swiftpm-testing-helper"))
+                  && CommandLine.arguments.contains { $0.contains("InstallTests") }
+              )
+        else {
+            return nil
+        }
+
         return try await fs.exists(atPath: swiftlyHomeBin) ? swiftlyHomeBin : nil
     }