Add build-time configuration for user's group list

Add a new build-time environment variable, USERADD_GROUPS, to allow the
list of supplementary groups a provisioned user is added to be tweaked
at built time.

libazureinit/build.rs

@@ -5,5 +5,7 @@ fn main() {
     // crates by `env!` macros.
     println!("cargo:rustc-env=PATH_HOSTNAMECTL=hostnamectl");
     println!("cargo:rustc-env=PATH_USERADD=useradd");
+    // The list of supplementary groups to add a provisioned user to.
+    println!("cargo:rustc-env=USERADD_GROUPS=adm,audio,cdrom,dialout,dip,floppy,lxd,netdev,plugdev,sudo,video");
     println!("cargo:rustc-env=PATH_PASSWD=passwd");
 }

libazureinit/src/provision/user.rs

@@ -37,7 +37,7 @@ fn useradd(name: &str) -> Result<(), Error> {
                       "Provisioning agent created this user based on username provided in IMDS",
                     )
                     .arg("--groups")
-                    .arg("adm,audio,cdrom,dialout,dip,floppy,lxd,netdev,plugdev,sudo,video")
+                    .arg(env!("USERADD_GROUPS"))
                     .arg("-d")
                     .arg(home_path)
                     .arg("-m")