bootstrap: Set shprog to bash on SunOS if available

[Vivianne Ombretta <vivianne%chinstrap.org@localhost>]

The system's sh is broken and results in a failed configure of
libarchive. This should not affect other systems as only the
SunOS case is modified.

diff --git a/bootstrap/bootstrap b/bootstrap/bootstrap
index e84e4dc2a..1ae955837 100755
--- a/bootstrap/bootstrap
+++ b/bootstrap/bootstrap
@@ -852,6 +852,7 @@ SunOS)
        if [ -x "/usr/bin/bash" ]; then
                bootstrap_sh=${SH:-/usr/bin/bash}
                bootstrap_sh_set=set
+               shprog="/usr/bin/bash"
        else
                need_mksh=yes
        fi