Fix 'pickString' evaluation

Similar to 'promptString', 'pickString' can be used to replace variables in task and
launch configurations. However 'pickString' values were not correctly handled which
prevented them from being evaluated. This is now fixed.

Fixes eclipse-theia#11585

Contributed on behalf of STMicroelectronics

packages/variable-resolver/src/browser/common-variable-contribution.ts

@@ -135,7 +135,8 @@ export class CommonVariableContribution implements VariableContribution {
                             });
                         }
                     }
-                    return this.quickInputService?.showQuickPick(elements, { placeholder: input.description });
+                    const selectedPick = await this.quickInputService?.showQuickPick(elements, { placeholder: input.description });
+                    return selectedPick?.value;
                 }
                 if (input.type === 'command') {
                     if (typeof input.command !== 'string') {