fix(gatsby-core-utils): multiple requests with different outputdir

[wardpeet]

Description

Checking cache with mutex is a bit tricky as you mostly need to check things twice if you don't want other tasks to wait for the mutex. Now when 2 requests with the same url come in before one is already done we check the cache too and copy or return the cached path.

[LekoArts]

Successfully published:

• babel-plugin-remove-graphql-queries@4.9.1
• babel-preset-gatsby@2.9.1
• gatsby-cli@4.9.1
• gatsby-core-utils@3.9.1
• gatsby-link@4.9.1
• gatsby-page-utils@2.9.1
• gatsby-plugin-benchmark-reporting@2.9.1
• gatsby-plugin-gatsby-cloud@4.9.1
• gatsby-plugin-image@2.9.1
• gatsby-plugin-manifest@4.9.1
• gatsby-plugin-mdx@3.9.1
• gatsby-plugin-offline@5.9.1
• gatsby-plugin-page-creator@4.9.1
• gatsby-plugin-preload-fonts@3.9.1
• gatsby-plugin-sharp@4.9.1
• gatsby-plugin-typescript@4.9.1
• gatsby-remark-images@6.9.1
• gatsby-source-contentful@7.7.2
• gatsby-source-drupal@5.9.1
• gatsby-source-filesystem@4.9.1
• gatsby-source-graphql@4.9.1
• gatsby-source-shopify@6.9.1
• gatsby-source-wordpress@6.9.1
• gatsby-telemetry@3.9.1
• gatsby-transformer-remark@5.9.1
• gatsby-transformer-sqip@4.9.1
• gatsby@4.9.3