tests: remove unnecessary variable interpolation

dvc.run does not expand variables. It is only done when it's actually read from dvc.yaml file.
So during dvc run, the shell will try to expand the variable, which does not exist.

I don't think that the intention was to use shell variables. But I don't think we should ever
use them, as they will fail on some shells (eg: fish) and keep the scripts as simple as possible.

tests/func/experiments/test_save.py

@@ -8,7 +8,7 @@ from dvc.scm import resolve_rev
 
 def setup_stage(tmp_dir, dvc, scm):
     tmp_dir.gen("params.yaml", "foo: 1")
-    dvc.run(name="echo-foo", outs=["bar"], cmd="echo ${foo} > bar")
+    dvc.run(name="echo-foo", outs=["bar"], cmd="echo foo > bar")
     scm.add(["dvc.yaml", "dvc.lock", ".gitignore", "params.yaml"])
     scm.commit("init")