Use custom install prefix.

--- a/utils/build-presets.ini
+++ b/utils/build-presets.ini
@@ -788,7 +788,7 @@
 install-swiftpm
 install-xctest
 install-libicu
-install-prefix=/usr
+install-prefix=%(install_prefix)s
 install-libcxx
 install-sourcekit-lsp
 build-swift-static-stdlib