Fix build failure on Linux (#304)

* Fix build failure on Linux

* Update Sources/Dependencies/DependencyValues.swift

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Co-authored-by: Stephen Celis <[email protected]>
Co-authored-by: Stephen Celis <[email protected]>

Author: finestructure

Sources/Dependencies/DependencyValues.swift

@@ -545,7 +545,7 @@ public final class CachedValues: @unchecked Sendable {
         case .preview:
           if !CachedValues.isAccessingCachedDependencies {
             value = CachedValues.$isAccessingCachedDependencies.withValue(true) {
-              #if compiler(>=6)
+              #if canImport(SwiftUI) && compiler(>=6)
                 return previewValues[key]
               #else
                 return Key.previewValue