Fix OpenBSD package install

.github/workflows/build.yml

diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index f6ca44c..aa8f535 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -81,7 +81,7 @@
         uses: vmactions/openbsd-vm@v1
         with:
           usesh: true
-          prepare: pkg_add pbzip2
+          prepare: /usr/sbin/pkg_add pbzip2
           run: |
             pwd
             sh ./test/run-makeself.sh