Pass stdin handle to backtick process

[casey]

Backtick processes are launched (via the brev crate) using Command::output, which by default prevents stdin from being inherited.

This diff passes stdin through. This allows backtick commands to read from stdin, and so fixes #376 and #374.

In particular, this allows:

password := `read PASSWORD && echo $PASSWORD`

ssh:
  echo {{password}}

To support this change, integration tests are modified to take an additional string argument, which is written to the just child process's stdin. Existing tests are passed "", and a few tests have been added that read from stdin and echo it, to make sure that it actually works.

cc: @slisznia

[casey]

It should be noted that the order that backticks execute in is unspecified and may change in the future.

Currently, dependencies between variables determine backtick execution order, not the order they appear in source. This justfile:

b := a + `echo bar`
a := `echo foo`

foo:
  echo {{b}}

will run echo foo before echo bar, even though echo foo appears a line after echo bar. However, this is an implementation detail, and may change.

So, if multiple top-level backticks read from stdin, the order that they read from stdin is undefined.