Use curl as default downloader for solvers

[tautschnig]

Switching from lwp-download to curl to solve problems with SSL
certificates on OS X (cf. #5169). These days, curl is available by
default on most systems anyway, and should thus be easy to use for
everyone. If need be, running make DOWNLOADER=lwp-download would yield
the previous behaviour.

Fixes: #5169

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Merging #5757 (2a5912c) into develop (d1e5b2c) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #5757   +/-   ##
========================================
  Coverage    69.65%   69.65%           
========================================
  Files         1243     1243           
  Lines       100843   100843           
========================================
  Hits         70243    70243           
  Misses       30600    30600           

Flag	Coverage Δ
cproversmt2	43.39% <ø> (ø)
regression	66.56% <ø> (ø)
unit	32.20% <ø> (ø)

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