npm: remove direct dependency on unused package

Our direct dependency on `@actions/exec` was removed in 274ad334e,
though we still depend on it indirectly.

common.js

@@ -3,7 +3,6 @@ const fs = require('fs');
 const path = require('path');
 const core = require('@actions/core');
 const github = require('@actions/github');
-const exec = require('@actions/exec');
 
 const VERSIONS_JSON = 'https://ziglang.org/download/index.json';
 const MACH_VERSIONS_JSON = 'https://pkg.machengine.org/zig/index.json';

package-lock.json

@@ -11,7 +11,6 @@
       "dependencies": {
         "@actions/cache": "^4.0.2",
         "@actions/core": "^1.11.1",
-        "@actions/exec": "^1.1.1",
         "@actions/github": "^6.0.1",
         "@actions/tool-cache": "^2.0.2",
         "sodium-native": "^4.3.3"

package.json

@@ -12,7 +12,6 @@
   "dependencies": {
     "@actions/cache": "^4.0.2",
     "@actions/core": "^1.11.1",
-    "@actions/exec": "^1.1.1",
     "@actions/github": "^6.0.1",
     "@actions/tool-cache": "^2.0.2",
     "sodium-native": "^4.3.3"