create_work: clarify error message

[davidpanderson]

You can't specify job cmdline args both in the input template and in the create_work command line.
Clarify this (at some point we could remove this restriction)

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 11.95%. Comparing base (df43b3f) to head (297ce35).
Report is 26 commits behind head on master.

Files with missing lines	Patch %	Lines
tools/process_input_template.cpp	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #6035      +/-   ##
============================================
+ Coverage     10.70%   11.95%   +1.24%     
  Complexity     1068     1068              
============================================
  Files           280      280              
  Lines         36713    36974     +261     
  Branches       8513     8533      +20     
============================================
+ Hits           3930     4420     +490     
+ Misses        32394    32154     -240     
- Partials        389      400      +11     

Files with missing lines	Coverage Δ
tools/process_input_template.cpp	0.00% <0.00%> (ø)

... and 4 files with indirect coverage changes