std/build: change default install prefix to zig-out

Currently the default install prefix is $BUILD_ROOT/zig-cache,
but mixing cache and artifacts makes little sense. Instead make
$BUILD_ROOT/zig-out the default.

Author: ifreund

.gitignore

@@ -10,6 +10,7 @@
 # -andrewrk
 
 zig-cache/
+zig-out/
 /release/
 /debug/
 /build/

lib/std/build.zig

@@ -195,7 +195,8 @@ pub const Builder = struct {
             self.install_prefix = install_prefix orelse "/usr";
             self.install_path = fs.path.join(self.allocator, &[_][]const u8{ dest_dir, self.install_prefix }) catch unreachable;
         } else {
-            self.install_prefix = install_prefix orelse self.cache_root;
+            self.install_prefix = install_prefix orelse
+                (fs.path.join(self.allocator, &[_][]const u8{ self.build_root, "zig-out" }) catch unreachable);
             self.install_path = self.install_prefix;
         }
         self.lib_dir = fs.path.join(self.allocator, &[_][]const u8{ self.install_path, "lib" }) catch unreachable;