Upgrade boost to 1.78

Latest subtree update pulled this in, and boost-url requires it.

Signed-off-by: Ed Tanous <edtanous@google.com>
Change-Id: Ice62443b588fae07bc7bebb4085c13a8373f9515
diff --git a/scripts/build-unit-test-docker b/scripts/build-unit-test-docker
index f3674e5..db8004a 100755
--- a/scripts/build-unit-test-docker
+++ b/scripts/build-unit-test-docker
@@ -91,7 +91,7 @@
         ],
     ),
     "boost": PackageDef(
-        rev="1.77.0",
+        rev="1.78.0",
         url=(
             lambda pkg, rev: f"https://downloads.yoctoproject.org/mirror/sources/{pkg}_{rev.replace('.', '_')}.tar.bz2"
         ),