Expand variables in the command to be run

[rojer]

I wanted to record some bits and pieces used in the version to embed in the binary later.
Tried to do the following:

```
run echo {revtime} > mos/version
run echo {revtime}+{git-commit}-xenial0 > mos/build_id
```

and learned that vars are not expanded in the command to run.
Would be nice if they were.

I can implement it and submit a patch, if there are no objections in principle.