Fix resolving of variables for che commands

[RomanNikitenko]

What does this PR do?

Allows to resolve variables for che commands before execution.
For example you can define command in your devfile:

- name: copy content
  actions:
  - type: exec
    component: che-dev
    command: >
              cp ${workspaceFolder}/.theia/tasks.json ${workspaceFolder}/cheCommands.json
    workdir: /projects

and ${workspaceFolder} will be replaced by /projects for example, or use ${file} to get path to
the file opened in the current editor

What issues does this PR fix or reference?

eclipse-che/che#14031

How to test

Use the component in your devfile

- 
    alias: theia-editor
    reference: >-
      https://raw.githubusercontent.com/RomanNikitenko/che-plugin-registry/master/v3/plugins/eclipse/che-theia/custom/meta.yaml
    type: cheEditor

The image contains:

• che-7.0.0 theia branch
• current 7.0.0 branch of che-theia + changes from the PR

Use a command with variables in your devfile, the simple example you can see above.

[AndrienkoAleksandr]

I want to test this branch.

[RomanNikitenko]

I want to test this branch.

@AndrienkoAleksandr
I'd appreciate it!
Do you need any help from my side for it?

[azatsarynnyy]

It would be nice to add resolving the variables in target.workingDir property as well. It's very helpful.

[RomanNikitenko]

It would be nice to add resolving the variables in target.workingDir property as well. It's very helpful.

maybe it's implemented on machine-exec level, because I see that variables are used for workingDir in devfiles for samples.

[AndrienkoAleksandr]

maybe it's implemented on machine-exec level, because I see that variables are used for workingDir in devfiles for samples.

che-machine-exec doesn't do such things.

[azatsarynnyy]

${CHE_PROJECTS_ROOT} it's not Theia's substitution variable but the environment variable

[RomanNikitenko]

@AndrienkoAleksandr @azatsarynnyy
thank you for clarification!

[RomanNikitenko]

@AndrienkoAleksandr @azatsarynnyy @olexii4
I added resolving of the variables for workingDir property after your review.

btw: I updated the images for testing (No pressure, just info for case if somebody wants to try it).

[AndrienkoAleksandr]

Tested, works fine.

[RomanNikitenko]

@slemeur
Can I merge the PR into 7.0.0 branch?
thank you!

[slemeur]

I'm +1 for it.
@l0rd