Add gcc-7 on 20.04

[ystefinko]

And use gcc-7 by specifying CC and CXX.

Relates-To: OLPEDGE-2776

Signed-off-by: Yaroslav Stefinko ext-yaroslav.stefinko@here.com

[codecov]

Codecov Report

Merging #1367 (886ed58) into master (b2a8e33) will increase coverage by 0.8%.
The diff coverage is n/a.

❗ Current head 886ed58 differs from pull request most recent head 92e9475. Consider uploading reports for the commit 92e9475 to get more accurate results

@@           Coverage Diff            @@
##           master   #1367     +/-   ##
========================================
+ Coverage    85.3%   86.2%   +0.8%     
========================================
  Files         337     380     +43     
  Lines       12953   13643    +690     
========================================
+ Hits        11053   11757    +704     
+ Misses       1900    1886     -14     

Impacted Files	Coverage Δ
...aservice-write/src/generated/model/LayerVersions.h	94.7% <0.0%> (-5.3%)	⬇️
olp-cpp-sdk-core/src/cache/DiskCacheEnv.cpp	72.7% <0.0%> (-5.1%)	⬇️
...sdk-dataservice-write/src/generated/PublishApi.cpp	41.3% <0.0%> (-1.0%)	⬇️
...dataservice-read/src/generated/api/MetadataApi.cpp	67.5% <0.0%> (-0.4%)	⬇️
...e/src/generated/serializer/IndexInfoSerializer.cpp	71.7% <0.0%> (-0.4%)	⬇️
...cpp-sdk-dataservice-read/src/CatalogClientImpl.cpp	93.2% <0.0%> (-0.2%)	⬇️
...p-sdk-dataservice-write/src/generated/IndexApi.cpp	41.8% <0.0%> (-0.1%)	⬇️
...k-dataservice-read/src/VolatileLayerClientImpl.cpp	94.3% <0.0%> (-0.1%)	⬇️
...dataservice-read/src/ProtectDependencyResolver.cpp	97.2% <0.0%> (-0.1%)	⬇️
...erated/serializer/UpdateIndexRequestSerializer.cpp	75.9% <0.0%> (-0.1%)	⬇️
... and 290 more

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