Explicitly install uidmap and run it all with set -x to see which com…

…mand errors

On appveyor env we got

	Selecting previously unselected package singularity-ce.
	(Reading database ... 300710 files and directories currently installed.)
	Preparing to unpack /tmp/singularity-ce.deb ...
	Unpacking singularity-ce (3.11.0-focal) ...
	dpkg: dependency problems prevent configuration of singularity-ce:
	 singularity-ce depends on uidmap; however:
	  Package uidmap is not installed.
	dpkg: error processing package singularity-ce (--install):
	 dependency problems - leaving unconfigured
	Processing triggers for man-db (2.9.1-1) ...
	Errors were encountered while processing:
	 singularity-ce
	Reading package lists... Done
	Building dependency tree
	Reading state information... Done
	Correcting dependencies... Done
	The following additional packages will be installed:
	  uidmap
	The following NEW packages will be installed:
	  uidmap
	0 upgraded, 1 newly installed, 0 to remove and 155 not upgraded.
	1 not fully installed or removed.
	Need to get 26.4 kB of archives.
	After this operation, 172 kB of additional disk space will be used.
	Do you want to continue? [Y/n] Abort.
	Command exited with code 1

tools/ci/install-singularity.sh

@@ -4,5 +4,7 @@ release="$(curl -fsSL https://api.github.com/repos/sylabs/singularity/releases/l
 codename="$(lsb_release -cs)"
 arch="$(dpkg --print-architecture)"
 wget -O /tmp/singularity-ce.deb https://github.com/sylabs/singularity/releases/download/$release/singularity-ce_${release#v}-${codename}_$arch.deb
+set -x
+sudo apt-get install uidmap
 sudo dpkg -i /tmp/singularity-ce.deb
 sudo apt-get install -f