chore(core): remove hardcoded runtime value in favor of user provided selection (#13285)

Currently Custom Resource Provider has `nodejs12.x` hardcoded. This commit will change this in favor of user selected runtime (only nodejs12 at this time).

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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: robertd

packages/@aws-cdk/core/lib/custom-resource-provider/custom-resource-provider.ts

@@ -85,7 +85,7 @@ export enum CustomResourceProviderRuntime {
   /**
    * Node.js 12.x
    */
-  NODEJS_12 = 'nodejs12'
+  NODEJS_12 = 'nodejs12.x'
 }
 
 /**
@@ -210,7 +210,7 @@ export class CustomResourceProvider extends CoreConstruct {
         MemorySize: memory.toMebibytes(),
         Handler: `${ENTRYPOINT_FILENAME}.handler`,
         Role: role.getAtt('Arn'),
-        Runtime: 'nodejs12.x',
+        Runtime: props.runtime,
         Environment: this.renderEnvironmentVariables(props.environment),
         Description: props.description ?? undefined,
       },