Detect yarn usage from execpath env variable as signal to use yarn

as the package manager. Otherwise it will use npm.

tasks/e2e-installs.sh

@@ -112,10 +112,11 @@ cd "$temp_app_path"
 npx create-react-app test-app-dist-tag --scripts-version=@latest
 cd test-app-dist-tag
 
-# Check corresponding scripts version is installed and no TypeScript is present.
+# Check corresponding scripts version is installed and no TypeScript or yarn is present by default
 exists node_modules/react-scripts
 ! exists node_modules/typescript
 ! exists src/index.tsx
+! exists yarn.lock
 exists src/index.js
 checkDependencies
 
@@ -133,16 +134,16 @@ grep '"version": "1.0.17"' node_modules/react-scripts/package.json
 checkDependencies
 
 # ******************************************************************************
-# Test --use-npm flag
+# Test --use-yarn flag
 # ******************************************************************************
 
 cd "$temp_app_path"
-npx create-react-app test-use-npm-flag --use-npm --scripts-version=1.0.17
-cd test-use-npm-flag
+npx create-react-app test-use-yarn-flag --use-yarn --scripts-version=1.0.17
+cd test-use-yarn-flag
 
 # Check corresponding scripts version is installed.
 exists node_modules/react-scripts
-[ ! -e "yarn.lock" ] && echo "yarn.lock correctly does not exist"
+exists yarn.lock
 grep '"version": "1.0.17"' node_modules/react-scripts/package.json
 checkDependencies
 
@@ -277,7 +278,7 @@ npm start -- --smoke-test
 # Test when PnP is enabled
 # ******************************************************************************
 cd "$temp_app_path"
-npx create-react-app test-app-pnp --use-pnp
+npx create-react-app test-app-pnp --use-yarn --use-pnp
 cd test-app-pnp
 ! exists node_modules
 exists .pnp.js

tasks/e2e-kitchensink-eject.sh

@@ -110,9 +110,6 @@ cd "$temp_app_path/test-kitchensink"
 # In kitchensink, we want to test all transforms
 export BROWSERSLIST='ie 9'
 
-# Link to test module
-npm link "$temp_module_path/node_modules/test-integrity"
-
 # ******************************************************************************
 # Finally, let's check that everything still works after ejecting.
 # ******************************************************************************
@@ -124,9 +121,6 @@ echo yes | npm run eject
 rm yarn.lock
 yarn add @babel/plugin-transform-react-jsx-source @babel/plugin-syntax-jsx @babel/plugin-transform-react-jsx @babel/plugin-transform-react-jsx-self
 
-# Link to test module
-npm link "$temp_module_path/node_modules/test-integrity"
-
 # Test the build
 REACT_APP_SHELL_ENV_MESSAGE=fromtheshell \
   NODE_PATH=src \