Call shouldGenerateSourceMap only once (#4104)

## Summary

This PR stores the result of `shouldGenerateSourceMap` to avoid calling
it twice.

## Test plan

Trust me bro

plugin/index.js

@@ -204,7 +204,9 @@ function buildWorkletString(t, fun, closureVariables, name, inputMap) {
 
   const code = generate(workletFunction).code;
 
-  if (shouldGenerateSourceMap()) {
+  const includeSourceMap = shouldGenerateSourceMap();
+
+  if (includeSourceMap) {
     // Clear contents array (should be empty anyways)
     inputMap.sourcesContent = [];
     // Include source contents in source map, because Flipper/iframe is not
@@ -216,8 +218,6 @@ function buildWorkletString(t, fun, closureVariables, name, inputMap) {
     }
   }
 
-  const includeSourceMap = shouldGenerateSourceMap();
-
   const transformed = transformSync(code, {
     plugins: [prependClosureVariablesIfNecessary()],
     compact: !includeSourceMap,