Revert "chore: newer pnpm version in CI"

This reverts commit 25156d7.

Author: Kelly Selden

.travis.yml

@@ -13,8 +13,7 @@ env:
   global:
   - DEBUG=ember-cli-update,boilerplate-update,git-diff-apply,git-fixtures
 before_install:
-# - curl -L https://unpkg.com/@pnpm/self-installer | node
-- npm i -g pnpm
+- curl -L https://unpkg.com/@pnpm/self-installer | node
 install:
 - pnpm install
 jobs:

appveyor.yml

@@ -14,8 +14,7 @@ init:
 # Install scripts. (runs after repo cloning)
 install:
   - ps: Install-Product node $env:nodejs_version
-  # - curl -L https://unpkg.com/@pnpm/self-installer | node
-  - npm i -g pnpm
+  - curl -L https://unpkg.com/@pnpm/self-installer | node
   - pnpm install
 
 # Post-install test scripts.