systemd: use qualified path

Inspired by
https://gerrit.openbmc.org/c/openbmc/phosphor-certificate-manager/+/54051

Tested: on real hardware, these binaries are in /usr/bin

Signed-off-by: Nan Zhou <nanzhou@google.com>
Change-Id: I7892cc0d416ca66bcda9ea6d58485dd66fb806a9
diff --git a/dist/bmc-vmi-ca-manager.service b/dist/bmc-vmi-ca-manager.service
index 83ef6ef..ec8ac84 100644
--- a/dist/bmc-vmi-ca-manager.service
+++ b/dist/bmc-vmi-ca-manager.service
@@ -2,7 +2,7 @@
 Description=BMC VMI CA authority manager
 
 [Service]
-ExecStart=/usr/bin/env bmc-vmi-ca
+ExecStart=/usr/bin/bmc-vmi-ca
 SyslogIdentifier=bmc-vmi-ca
 Restart=always
 UMask=0007