eval substitution errors

Bug report

Required Info:

• Operating System:
  Ubuntu 20.04
• Installation type:
  Binaries
• Version or commit hash:
  foxy latest release
• DDS implementation:
  Fast-RTPS
• Client library (if applicable):
  N/A

Steps to reproduce issue

Create and launch a xml launch file like this

<launch>
  <let name="variable" value="val1"/>
  <node pkg="demo_nodes_cpp" exec="talker if="$(eval variable == 'val1')" output="screen" />
</launch>

Expected behavior

talker starts running

Actual behavior

I get this error: TypeError: eval substitution expects 1 argument

Additional information

It seems that spaces confuse the eval substitution. Even if I remove the spaces, launch fails with

launch.invalid_launch_file_error.InvalidLaunchFileError: Caught exception when trying to load file of format [xml]: No terminal defined for ''' at line 1 col 20

$(eval variable=='val1')
                 ^

Expecting: {'UNQUOTED_RSTRING', 'SPACE', 'RPAR', 'DOLLAR'}

which I guess means that it doesn't like the single quote character

Start by reproducing the XML launch example with the eval substitution, including spaces and single quotes, and inspect the launch substitution and XML parsing entry points involved. Done means the expression parses without the reported errors and the demo_nodes_cpp talker starts as expected.