policy: enable proxy network from source policy caps

Evaluate source policy caps before solve requests so policies can enable
BuildKit proxy networking. Policy can return caps {"exec.proxy": true}
during the caps request to enable proxy network
support for the solve.

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

build/build.go

@@ -140,6 +140,7 @@ type policyFileSpec struct {
 type policyEvalOpt struct {
 	Strict   bool
 	LogLevel *logrus.Level
+	SkipCaps bool
 }
 
 type policyOpt struct {