Make use of SHELL environment variable

[everops-mat]

I typically use rc shell, but saml2aws exec uses /bin/sh -c '....' which causes some problems. I have a small PR that has exec pull the SHELL environment variable and us it if it exists, otherwise uses /bin/sh

#862

[gliptak]

an alternative approach might be to look up /bin/bash and fallback to /bin/sh