makefile and shell scripts could check for SF_DEV_INC_SH=true

[andreineculau]

this variable is set in https://github.com/tobiipro/support-firecloud/blob/master/sh/dev.inc.sh#L24 in order to signal that the shell session is adequately set up.

when running make in a repository set up with support-firecloud or call a support-firecloud script like https://github.com/tobiipro/support-firecloud/tree/master/bin , we could see if SF_DEV_INC_SH=true and crash early if it's not.

this would be a quick feedback loop that major things are not set up adequately on the development machine and thus the execution may have unexpected side-effects

PS: a solution is needed for the CI flow (where there's no SF_DEV_INC_SH environment variable). Solutions range

• from just exporting SF_DEV_INC_SH=true
• to implement the check as SF_DEV_INC_SH=true || CI=true,
• to implement a new environment variable in repo/dot.ci.sh.sf like SF_CI_SH and implement the check as SF_DEV_INC_SH=true || SF_CI_SH=true