fix: pass environment to workers

Passes the environment through to workers, allowing more control by the
user of Bakelite. In particular, this allows the GOCACHE and GO111MODULE
environment variables to be configured.

The CGO_ENABLED, GOOS, and GOARCH environment variables are still
controlled by Bakelite, as before.

main.go

@@ -124,6 +124,8 @@ See also: go build, go install, go clean.
 
 	platforms := plBuilder.Build()
 
+	environ := parseEnvironment(os.Environ())
+
 	var (
 		parallelJobs = runtime.NumCPU()
 		sem          = semaphore.NewWeighted(int64(parallelJobs))
@@ -143,7 +145,7 @@ See also: go build, go install, go clean.
 
 			go func(platform Platform, pkg string) {
 				defer sem.Release(1)
-				err := build(ctx, platform, pkg)
+				err := build(ctx, environ, platform, pkg)
 
 				if err != nil {
 					errored = true
@@ -162,20 +164,21 @@ See also: go build, go install, go clean.
 	}
 }
 
-func build(ctx context.Context, platform Platform, pkg string) error {
+func build(ctx context.Context, environ kvs, platform Platform, pkg string) error {
 	name := fmt.Sprintf("%s-%s-%s", filepath.Base(pkg), platform.OS, platform.Arch)
 
 	if platform.OS == OS_WINDOWS {
 		name += ".exe"
 	}
 
-	env := kvs{
-		"GOOS":   string(platform.OS),
-		"GOARCH": string(platform.Arch),
-		"GOROOT": os.Getenv("GOROOT"),
-		"GOPATH": os.Getenv("GOPATH"),
+	env := kvs{}
+	for key, val := range environ {
+		env[key] = val
 	}
 
+	env["GOOS"] = string(platform.OS)
+	env["GOARCH"] = string(platform.Arch)
+
 	if cgo {
 		env["CGO_ENABLED"] = "1"
 	} else {
@@ -244,3 +247,13 @@ func parsePlatforms(plBuilder *PlatformBuilder, fields []string) *PlatformBuilde
 
 	return plBuilder
 }
+
+func parseEnvironment(environ []string) kvs {
+	env := kvs{}
+	for _, s := range environ {
+		split := strings.SplitN(s, "=", 2)
+		env[split[0]] = split[1]
+	}
+
+	return env
+}