ci: Work around npm behavior changes to fix CI on main (#206)

* ci: Prevent recursive prepare script invocations

A change between npm 8.9 and 8.10 started running the main package's
`prepare` script as part of the examples' installs. This broke CI when
the install step launched a fork bomb and got killed after several
minutes.

This change should fix builds on `main`.

* ci: Link with legacy peer deps

Something about peer dependencies changed between npm 8.5.5 and 8.6.0
that is causing CI to fail on `main` with ESLint 6 on the test matrix.

This works around the issue.

npm-prepare.js

@@ -6,17 +6,23 @@
 
 "use strict";
 
-const childProcess = require("child_process");
-const fs = require("fs");
-const path = require("path");
+if (!process.env.NO_RECURSIVE_PREPARE) {
+    const childProcess = require("child_process");
+    const fs = require("fs");
+    const path = require("path");
 
-const examplesDir = path.resolve(__dirname, "examples");
-const examples = fs.readdirSync(examplesDir)
-    .filter(exampleDir => fs.statSync(path.join(examplesDir, exampleDir)).isDirectory())
-    .filter(exampleDir => fs.existsSync(path.join(examplesDir, exampleDir, "package.json")));
+    const examplesDir = path.resolve(__dirname, "examples");
+    const examples = fs.readdirSync(examplesDir)
+        .filter(exampleDir => fs.statSync(path.join(examplesDir, exampleDir)).isDirectory())
+        .filter(exampleDir => fs.existsSync(path.join(examplesDir, exampleDir, "package.json")));
 
-for (const example of examples) {
-    childProcess.execSync("npm install", {
-        cwd: path.resolve(examplesDir, example)
-    });
+    for (const example of examples) {
+        childProcess.execSync("npm install", {
+            cwd: path.resolve(examplesDir, example),
+            env: {
+                ...process.env,
+                NO_RECURSIVE_PREPARE: "true"
+            }
+        });
+    }
 }

tests/lib/plugin.js

@@ -82,7 +82,7 @@ describe("recommended config", () => {
                 // eslint-disable-next-line no-invalid-this
                 this.timeout(30000);
 
-                execSync("npm link && npm link eslint-plugin-markdown");
+                execSync("npm link && npm link eslint-plugin-markdown --legacy-peer-deps");
             } else {
                 throw error;
             }