Allow concatenation on environment variables


Description of the feature request

I would love having a way to concatenate text to an environment variable.
This could be done by having a “terminal” char on the environment variable.

Use case / for what or how I would use it

Currently if we are defining a VARIANT and we try to use it on a custom gradle runner step the support is quite limited. Ie I can use the VARIANT when the gradle tasks finish on it, but the replacement would not work if there are some extra text added.
- gradle_task: assemble$VARIANT assemble$VARIANTAndroidTest
So, the first one gets replaced as expected, but the second one does not work and it is running assemble.

If there was a terminal char I could ideally write something like:
- gradle_task: assemble${VARIANT} assemble${VARIANT}AndroidTest
Avoiding the need to create 3 different environment variables that represent the same thing. I know I can create them with a custom step, but it does not feel right to me.