cmake: restore cmake args list in buildinfo.txt

[vszakats]

This feature was recently dropped because of a bad side-effect of
silencing unused cmake command-line option warnings.

Fix this issue by retrieving variable values using get_property(),
instead of accessing the variables directly. It allows restoring
this feature without the bad side-effect.

Also limit the logic to CI runs.

Follow-up to 96edb5f #15501

[vszakats]

/cc @dg0yt

[dg0yt]

Did you test how much this writes on CMake re-run? I would assume it includes all variables which are also in CMakeCache.txt, i.e. all the cache variables from find_library and friends.

[vszakats]

The only place the result appears is when CI is detected, and re-runs are typically not done there. The unnecessary work is still bad, so how about I'd limit this to CI runs like already done for buildinfo.txt generation?

If still not good we might limit it further to curl CI runs.

[vszakats]

Running an initial out-of-tree cmake -B _bld <options>, then re-running cmake -B _bld shows a subset of the original command-line option, but not the whole cache:

initial:
-- | -DBUILD_LIBCURL_DOCS="OFF" -DBUILD_MISC_DOCS="OFF" -DCMAKE_TRY_COMPILE_TARGET_TYPE="STATIC_LIBRARY" -DCMAKE_UNITY_BUILD="ON" -DCURL_BROTLI="ON" -DCURL_CA_BUNDLE="none" -DCURL_CA_SEARCH_SAFE="ON" -DCURL_DISABLE_LDAP="ON" -DENABLE_CURL_MANUAL="OFF" -DOPENSSL_ROOT_DIR="/usr/local/opt/openssl@3" -DUNUSED_VARIABLE="FOOBAR"|

re-run (manually aligned with the initial run):
-- |                                                    -DCMAKE_TRY_COMPILE_TARGET_TYPE="STATIC_LIBRARY" -DCMAKE_UNITY_BUILD="ON"                                            -DCURL_CA_SEARCH_SAFE="ON"                                                     -DOPENSSL_ROOT_DIR="/usr/local/opt/openssl@3" -DUNUSED_VARIABLE="FOOBAR"|

[vszakats]

I plan to merge this, unless there is some remaining concern.