conditionally escape rule builder command

For rules which use RuleBuilder with Ninja variables (using the
BuildWithUnescapedNinjaVars function) but are not sandboxed, the command
output by RuleBuilder must not be escaped, or else Ninja variables will
not be evaluated until they are run on the command line.

Test: m libnum_traits liboid_registry crosvm droid
Change-Id: I81611368ab1975220f7f517df2f57646deac2af8