Ignore user's `.bash_profile`

[bryphe]

For esy, with the bootstrapped builds, we hit issues where the users .bash_profile (which initializes the OPAM repo), can collide with our build commands - bringing in parts of the OPAM environment that break the build.

We should have an option or a way to pass-through arguments to bash, to ignore the .bash_profile.

[bryphe]

It seems the HOME environment variable overrides the default. We should perhaps set up the /etc/nsswitch.conf to have a specific hard-coded home path. https://superuser.com/questions/942908/changing-the-home-for-cygwin

In addition, we are using the -l flag for spinning up our bash process - perhaps this is not needed?