clang19 manifest: add CLANG19_PREINSTALLED preinstalled.env opt-out

Summary:
Hosts that bring their own clang-19 toolchain (notably the manylinux CI
container, which installs clang via an LLVM-repo workflow step before
getdeps runs) need a way to tell getdeps "the binary you're looking for
is already on PATH, don't try to install it via deb/rpm".

Add a [preinstalled.env] section listing CLANG19_PREINSTALLED. Setting
that env var to any value makes the manifest report
_is_satisfied_by_preinstalled_environment() = True and skip system-
package install entirely.

Reviewed By: bigfootjon

Differential Revision: D104428966

fbshipit-source-id: 57b15cc95314949f3cb79354c2540cbe63ec634e

Author: r-barnes

build/fbcode_builder/manifests/clang19

@@ -1,6 +1,13 @@
 [manifest]
 name = clang19
 
+# Allow callers to claim clang19 is already on PATH by setting
+# CLANG19_PREINSTALLED=1 in the environment. Used by hosts that bring
+# their own toolchain (e.g. the manylinux CI container which installs
+# clang via a workflow pre-step).
+[preinstalled.env]
+CLANG19_PREINSTALLED
+
 [debs.os=linux]
 clang-19