Revert "Fix #7296:"

This broke build scripts that wanted to refer to `exe_dir` or
`install_path`.

There has also been some pushback and discussion on this breaking
change. I think it should be re-evaluated.

This reverts commit a1a1929.

lib/std/build.zig

@@ -200,16 +200,12 @@ pub const Builder = struct {
             const install_prefix = self.install_prefix orelse "/usr";
             self.install_path = fs.path.join(self.allocator, &[_][]const u8{ dest_dir, install_prefix }) catch unreachable;
         } else {
-            self.install_path = self.install_prefix orelse blk: {
-                const p = if (self.release_mode) |mode| switch (mode) {
-                    .Debug => "debug",
-                    .ReleaseSafe => "release",
-                    .ReleaseFast => "release",
-                    .ReleaseSmall => "release",
-                } else "debug";
+            const install_prefix = self.install_prefix orelse blk: {
+                const p = self.cache_root;
                 self.install_prefix = p;
-                break :blk self.pathFromRoot(p);
+                break :blk p;
             };
+            self.install_path = install_prefix;
         }
         self.lib_dir = fs.path.join(self.allocator, &[_][]const u8{ self.install_path, "lib" }) catch unreachable;
         self.exe_dir = fs.path.join(self.allocator, &[_][]const u8{ self.install_path, "bin" }) catch unreachable;

lib/std/special/build_runner.zig

@@ -134,8 +134,8 @@ pub fn main() !void {
         }
     }
 
-    try runBuild(builder);
     builder.resolveInstallPrefix();
+    try runBuild(builder);
 
     if (builder.validateUserInputDidItFail())
         return usageAndErr(builder, true, stderr_stream);