hostinfo: move packageType out to platform-specific files

Change-Id: I3236b3d4e2376dd7e2482c2562817b1b6f44872e
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

hostinfo/hostinfo_linux.go

@@ -21,6 +21,7 @@ import (
 
 func init() {
 	osVersion = osVersionLinux
+	packageType = packageTypeLinux
 
 	if v := linuxDeviceModel(); v != "" {
 		SetDeviceModel(v)
@@ -115,3 +116,13 @@ func osVersionLinux() string {
 	}
 	return fmt.Sprintf("Other%s", attr)
 }
+
+func packageTypeLinux() string {
+	// Report whether this is in a snap.
+	// See https://snapcraft.io/docs/environment-variables
+	// We just look at two somewhat arbitrarily.
+	if os.Getenv("SNAP_NAME") != "" && os.Getenv("SNAP") != "" {
+		return "snap"
+	}
+	return ""
+}