Allow overriding build triple via flag.

We first check the configuration, then passed parameters (--build), then
fall back to the auto-detection that bootstrap.py does.

Fixes #39673.

src/bootstrap/config.rs

@@ -290,7 +290,6 @@ impl Config {
         config.docs = true;
         config.rust_rpath = true;
         config.rust_codegen_units = 1;
-        config.build = flags.build;
         config.channel = "dev".to_string();
         config.codegen_tests = true;
         config.rust_dist_src = true;
@@ -319,6 +318,11 @@ impl Config {
 
         let build = toml.build.clone().unwrap_or(Build::default());
         set(&mut config.build, build.build.clone().map(|x| INTERNER.intern_string(x)));
+        set(&mut config.build, flags.build);
+        if config.build.is_empty() {
+            // set by bootstrap.py
+            config.build = INTERNER.intern_str(&env::var("BUILD").unwrap());
+        }
         config.hosts.push(config.build.clone());
         for host in build.host.iter() {
             let host = INTERNER.intern_str(host);

src/bootstrap/flags.rs

@@ -33,7 +33,7 @@ pub struct Flags {
     pub on_fail: Option<String>,
     pub stage: Option<u32>,
     pub keep_stage: Option<u32>,
-    pub build: Interned<String>,
+    pub build: Option<Interned<String>>,
 
     pub host: Vec<Interned<String>>,
     pub target: Vec<Interned<String>>,
@@ -327,9 +327,7 @@ Arguments:
             stage: stage,
             on_fail: matches.opt_str("on-fail"),
             keep_stage: matches.opt_str("keep-stage").map(|j| j.parse().unwrap()),
-            build: INTERNER.intern_string(matches.opt_str("build").unwrap_or_else(|| {
-                env::var("BUILD").unwrap()
-            })),
+            build: matches.opt_str("build").map(|s| INTERNER.intern_string(s)),
             host: split(matches.opt_strs("host"))
                 .into_iter().map(|x| INTERNER.intern_string(x)).collect::<Vec<_>>(),
             target: split(matches.opt_strs("target"))