change env variable from YARN_CACHE to USE_YARN_CACHE

[danielleadams]

This was causing a bug with Yarn 2 when YARN_CACHE was set, which is a reserved env var value. This will also output USE_YARN_CACHE at the beginning of the build with the other user-configured build settings.

Blocks #795

Error from Yarn 2 logs

-----> Creating runtime environment

       NPM_CONFIG_LOGLEVEL=error
       YARN_CACHE=true
       NODE_VERBOSE=false
       NODE_ENV=production
       NODE_MODULES_CACHE=true

-----> Installing binaries
       engines.node (package.json):  unspecified
       engines.npm (package.json):   unspecified (use default)
       engines.yarn (package.json):  unspecified (use default)

       Resolving node version 12.x...
       Downloading and installing node 12.18.2...
       Using default npm version: 6.14.5
       Resolving yarn version 1.x...
       Downloading and installing yarn (1.22.4)...
       Using yarn 2.0.0-rc.33

-----> Installing dependencies
       Using Yarn 2, skipping installations.

-----> Build
       Running build (yarn)
       Usage Error: Unrecognized or legacy configuration settings found: cache - run "yarn config -v" to see the list of settings supported in Yarn (in <environment>)

       $ yarn run [--inspect] [--inspect-brk] <scriptName> ...