Config inject doesn't work when must_exist == true.

[rcannood]

Example config.vsh.yaml:

functionality:
  name: must_exist
  arguments:
    - name: "--input"
      type: file
      must_exist: true
      example: foo.txt
  resources:
    - type: r_script
      path: script.R

Running config inject results in:

$ viash config inject config.vsh.yaml
[error] File 'foo.txt' does not exist.

Normally this error would be expected when running viash run config.vsh.yaml --input non_exist.txt if non_exist.txt does not exist. In the context of a config inject, this error is not expected.

[rcannood]

Solved by #274