phosphor-i2cvr-software-update: move to libexec

Non-user executables are suppose to be installed into libexec[1].

[1]: https://github.com/openbmc/docs/blob/master/anti-patterns.md#incorrect-placement-of-executables-in-sbin-usrsbin-or-bin-usrbin

Tested:

```
$ meson install -C builddir --destdir /tmp/phosphor-bmc-code-mgmt
$ find /tmp/phosphor-bmc-code-mgmt/ -name phosphor-i2cvr-software-update
/tmp/phosphor-bmc-code-mgmt/usr/local/libexec/phosphor-bmc-code-mgmt/phosphor-i2cvr-software-update
```

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I87fded12f5bafd5deb5b130f6f0350d67ce401b3
diff --git a/i2c-vr/xyz.openbmc_project.Software.I2CVR.service b/i2c-vr/xyz.openbmc_project.Software.I2CVR.service
index 6d129ad..e36111b 100644
--- a/i2c-vr/xyz.openbmc_project.Software.I2CVR.service
+++ b/i2c-vr/xyz.openbmc_project.Software.I2CVR.service
@@ -7,7 +7,7 @@
 Restart=always
 Type=exec
 RemainAfterExit=no
-ExecStart=/usr/bin/phosphor-i2cvr-software-update
+ExecStart=/usr/libexec/phosphor-code-mgmt/phosphor-i2cvr-software-update
 
 [Install]
 WantedBy=multi-user.target