fix(deps): frozen installation fails during ugprade workflow (#801)

Fixes #800.

The problem was that projen itself can change `package.json` files due to its own requirements. 

> For example https://github.com/cdklabs/cdk-triggers/blob/main/.projenrc.js#L6

Which causes the `package.json` file to change after running `yarn install` - and a subsequent frozen installation would fail. 

This PR uses projen itself to install the dependencies and update the lock file, so this happens after all `package.json` mutations have happened. 

---

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

.projen/tasks.json

@@ -138,12 +138,15 @@
     },
     "upgrade-dependencies": {
       "name": "upgrade-dependencies",
+      "env": {
+        "CI": "0"
+      },
       "steps": [
         {
           "exec": "npm-check-updates --upgrade --target=minor --reject='projen'"
         },
         {
-          "exec": "yarn install --check-files"
+          "exec": "/bin/bash ./projen.bash"
         }
       ]
     },

API.md

@@ -2870,19 +2870,6 @@ removeScript(name: string): void
 
 
 
-#### renderInstallCommand(frozen)🔹 <a id="projen-nodepackage-renderinstallcommand"></a>
-
-
-
-```ts
-renderInstallCommand(frozen: boolean): string
-```
-
-* **frozen** (<code>boolean</code>)  *No description*
-
-__Returns__:
-* <code>string</code>
-
 #### setScript(name, command)🔹 <a id="projen-nodepackage-setscript"></a>
 
 Override the contents of an npm package.json script.