Upgrading lcov install to install v1.15 to be compatible with GCC9 (#46847)
Summary:
According to [this issue](https://github.com/linux-test-project/lcov/issues/58), LCOV 1.14 and below are not compatible with GCC9 when gathering coverage.
Instead of installing `lcov` with `apt-get`, which installs version 1.13, this PR would install v1.15 from source onto the ubuntu Docker images.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/46847
Reviewed By: seemethere
Differential Revision: D24540444
Pulled By: janeyx99
fbshipit-source-id: 0ac2a37241d94cdd8fea2fded7984c495a64cedc
