OvmfPkg: Fix the build.sh shebang line to avoid depending on location of bash

OvmfPkg: Fix the build.sh shebang line to avoid depending on location of bash

The bash binary can be in various locations depending on the system: on Linux
it's in /bin while on BSD it's normally in /usr/local/bin. However, the
env binary is almost always in /usr/bin and so can be used to find and start
the shell.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Bruce Cran <bruce@cran.org.uk>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@17883 6f19259b-4bc3-4df7-8a09-765794883524