CI: use special environment var in GHA for Windows' PATH

[lukaszstolarczuk]

The old 'addPath' is depreciated now. For more details see:
https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/

mea culpa, this is a merge back from #975 - this should have been based on the oldest branch with GHA enabled, from the beginning.

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This change is 

[codecov]

Codecov Report

Merging #981 (83fa6fa) into stable-1.9 (db5da1b) will increase coverage by 1.17%.
The diff coverage is n/a.

@@              Coverage Diff               @@
##           stable-1.9     #981      +/-   ##
==============================================
+ Coverage       92.16%   93.34%   +1.17%     
==============================================
  Files              40       40              
  Lines            3792     3815      +23     
==============================================
+ Hits             3495     3561      +66     
+ Misses            297      254      -43     

Flag	Coverage Δ
tests_clang_debug_cpp17	80.63% <ø> (+9.00%)	⬆️
tests_gcc_debug	93.74% <ø> (+0.08%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...ude/libpmemobj++/container/concurrent_hash_map.hpp	94.08% <0.00%> (-0.07%)	⬇️
include/libpmemobj++/container/array.hpp	100.00% <0.00%> (ø)
include/libpmemobj++/detail/temp_value.hpp	100.00% <0.00%> (ø)
...lude/libpmemobj++/make_persistent_array_atomic.hpp	100.00% <0.00%> (ø)
include/libpmemobj++/container/basic_string.hpp	96.64% <0.00%> (+0.31%)	⬆️
include/libpmemobj++/detail/common.hpp	88.23% <0.00%> (+0.35%)	⬆️
include/libpmemobj++/container/segment_vector.hpp	95.22% <0.00%> (+0.74%)	⬆️
include/libpmemobj++/container/vector.hpp	93.51% <0.00%> (+5.60%)	⬆️
include/libpmemobj++/make_persistent_atomic.hpp	100.00% <0.00%> (+9.09%)	⬆️
include/libpmemobj++/timed_mutex.hpp	87.09% <0.00%> (+32.25%)	⬆️
... and 1 more

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