Hoist call to ImmutableMap.copyOf().

RELNOTES: None.
PiperOrigin-RevId: 210161137
diff --git a/src/main/java/com/google/devtools/build/lib/skyframe/SkyframeActionExecutor.java b/src/main/java/com/google/devtools/build/lib/skyframe/SkyframeActionExecutor.java
index 56154e4..e466704 100644
--- a/src/main/java/com/google/devtools/build/lib/skyframe/SkyframeActionExecutor.java
+++ b/src/main/java/com/google/devtools/build/lib/skyframe/SkyframeActionExecutor.java
@@ -361,7 +361,8 @@
   }
 
   public void setClientEnv(Map<String, String> clientEnv) {
-    this.clientEnv = clientEnv;
+    // Copy once here, instead of on every construction of ActionExecutionContext.
+    this.clientEnv = ImmutableMap.copyOf(clientEnv);
   }
 
   boolean usesActionFileSystem() {