Make runner shell known to custom executor
Description
The executor implementations which gitlab-runner supports out-of-the-box, all know about the runner shell in order to know how to run the rendered build script. Custom executors do not know about the shell and either have to be configured separatly or employ an oracle which parses the scripts in order to detect the used shell. In order to avoid configuring my executor manually with information i have already provided to the runner, i would like to know which shell script renderer (sh. bash, powershell, cmd) was used.
Proposal
The most straight forward solution would be to add a new predefined environment variable with the value of the shell renderer. not the actual commandline to execute the script:
CI_SHELL (sh. bash, pwsh, powershell, cmd)