fix(ci): use npm@6 (#603)

Refs: https://twitter.com/addaleax/status/1347909082829750273

Author: addaleax

.evergreen/.install_node

@@ -15,7 +15,7 @@ else
 
   nvm install $NODE_JS_VERSION
   nvm alias default $NODE_JS_VERSION
-  npm install -g npm@latest
+  npm install -g npm@6
 fi
 
 . "$BASEDIR/.setup_env"

.evergreen/InstallNode.ps1

@@ -23,4 +23,4 @@ Remove-Item .\npm.cmd
 Remove-Item .\npx
 Remove-Item .\npx.cmd
 Move-Item .\node_modules\npm -Destination .\node_modules\npm2
-.\node.exe .\node_modules\npm2\bin\npm-cli.js i -g npm@latest
+.\node.exe .\node_modules\npm2\bin\npm-cli.js i -g npm@6