semaphore: switch to another keyserver

lxc-create has been failing to download the image today with
```
+ sudo lxc-create -n buster-amd64 -t download -- -d debian -r buster -a amd64
Setting up the GPG keyring
ERROR: Unable to fetch GPG key from keyserver
lxc-create: buster-amd64: lxccontainer.c: create_run_template: 1617 Failed to create container from template
lxc-create: buster-amd64: tools/lxc_create.c: main: 327 Failed to create container buster-amd64
```
Let's try another keyserver.

semaphoreci/semaphore-runner.sh

@@ -18,7 +18,7 @@ create_container() {
     # create autopkgtest LXC image; this sometimes fails with "Unable to fetch
     # GPG key from keyserver", so retry a few times
     for retry in $(seq 5); do
-        sudo lxc-create -n $CONTAINER -t download -- -d $DISTRO -r $RELEASE -a $ARCH && break
+        sudo lxc-create -n $CONTAINER -t download -- -d $DISTRO -r $RELEASE -a $ARCH --keyserver hkp://keyserver.ubuntu.com:80 && break
         sleep $((retry*retry))
     done