env dependency ignores vars which dont use backticks

[jayvdb]

Current algorithm doesnt see that c must be deferred until after a

a=`echo foo`
b={env:a}
c=`echo {env:b}`

[jayvdb]

Couldnt reproduce with:

[testenv:indirect]
setenv =
  FOO=`python -c 'import sys; sys.stdout.write("foo")'`
  BAR={env:FOO}
  BAZ=`python -c 'import sys; sys.stdout.write("{env:BAR}")'`
commands =
  python -c 'assert "{env:BAZ}" == "foo"'

[jayvdb]

It does occur on py27, so likely it is passing on py37 due to dict ordering.

[jayvdb]

A real solution has dragons everywhere. The regex is the problem, as it doesnt allow identification of FOO in {env:FOO:{env:BAR}}.

If I dont find a good fix for that soon, I'll go with my existing solution which ignores that, or add a second make-shift dodgy regex which can handle those some of the time.