Fix release workflow bumping the v1 branch

• It needs a full checkout.

Add a working-directory input to resolve .ruby-version, Gemfile.lock, etc

See #50

Do not show a "Installing Bundler" group for bundler: none

v1.31.1

Document the consequences of using a specific version or commit sha

Use grouping in output and measure each group

• See MSP-Greg#3

Do no install Bundler on ruby-head/mingw/mswin

• Since latest Bundler is not necessarily compatible with ruby-head, and the stdlib Bundler should be recent enough for ruby-head
• See #48

Add latest CRuby release on Windows

• Fixes #46

Install Bundler from Gemfile.lock BUNDLED WITH or install latest

• Fixes #38

Use builds with --enable-shared and add new Ruby releases

• Fixes #43
• Add Ruby 2.4.10, 2.5.8, 2.6.6 and 2.7.1 on Ubuntu & macOS (#46).
• Install Bundler at runtime for more flexibility

Set MAKE on Windows

• See #45

Automatically create a ~/.gemrc file

• To avoid generating documentation on gem install, which can take a long time.