(WIP) Support valueFrom syntax for build env vars (openshift#14143)

Trello card: https://trello.com/c/lDxfG8GO/1062-8-support-valuefrom-syntax-for-build-env-vars-techdebt

pkg/build/api/v1/types.go

@@ -584,7 +584,6 @@ type CustomBuildStrategy struct {
 	PullSecret *kapi.LocalObjectReference `json:"pullSecret,omitempty" protobuf:"bytes,2,opt,name=pullSecret"`
 
 	// env contains additional environment variables you want to pass into a builder container.
-	// ValueFrom is not supported.
 	Env []kapi.EnvVar `json:"env,omitempty" protobuf:"bytes,3,rep,name=env"`
 
 	// exposeDockerSocket will allow running Docker commands (and build Docker images) from
@@ -641,7 +640,6 @@ type DockerBuildStrategy struct {
 	NoCache bool `json:"noCache,omitempty" protobuf:"varint,3,opt,name=noCache"`
 
 	// env contains additional environment variables you want to pass into a builder container.
-	// ValueFrom is not supported.
 	Env []kapi.EnvVar `json:"env,omitempty" protobuf:"bytes,4,rep,name=env"`
 
 	// forcePull describes if the builder should pull the images from registry prior to building.
@@ -677,7 +675,6 @@ type SourceBuildStrategy struct {
 	PullSecret *kapi.LocalObjectReference `json:"pullSecret,omitempty" protobuf:"bytes,2,opt,name=pullSecret"`
 
 	// env contains additional environment variables you want to pass into a builder container.
-	// ValueFrom is not supported.
 	Env []kapi.EnvVar `json:"env,omitempty" protobuf:"bytes,3,rep,name=env"`
 
 	// scripts is the location of Source scripts
@@ -718,7 +715,6 @@ type JenkinsPipelineBuildStrategy struct {
 	Jenkinsfile string `json:"jenkinsfile,omitempty" protobuf:"bytes,2,opt,name=jenkinsfile"`
 
 	// env contains additional environment variables you want to pass into a build pipeline.
-	// ValueFrom is not supported.
 	Env []kapi.EnvVar `json:"env,omitempty" protobuf:"bytes,3,rep,name=env"`
 }
 
@@ -1062,7 +1058,6 @@ type BuildRequest struct {
 	LastVersion *int64 `json:"lastVersion,omitempty" protobuf:"varint,6,opt,name=lastVersion"`
 
 	// env contains additional environment variables you want to pass into a builder container.
-	// ValueFrom is not supported.
 	Env []kapi.EnvVar `json:"env,omitempty" protobuf:"bytes,7,rep,name=env"`
 
 	// triggeredBy describes which triggers started the most recent update to the