builder: fix interpolation for build_steps

When the build_steps alternative for inputting boot commands was merged,
we did not add an exception for interpolating its contents prior to the
type_boot_command step.

This caused all its contents to be pre-interpolated, but since it relies
on data that was not populated at interpolation-time, we ended-up with
the templated content replaced by <no value>.

Adding the boot_steps to the exceptions fixes this problem.

builder/qemu/config.go

@@ -632,6 +632,7 @@ func (c *Config) Prepare(raws ...interface{}) ([]string, error) {
 		InterpolateFilter: &interpolate.RenderFilter{
 			Exclude: []string{
 				"boot_command",
+				"boot_steps",
 				"qemuargs",
 			},
 		},