[File] disambiguate bfs::exist constructor

[gicmo]

Explicitly instantiate a boost::filesystem::path object to avoid any ambiguity via implicit type conversion; as seems to be happen with boost 1.71:

/src/src/File.cpp: In static member function ‘static nix::File nix::File::open(const string&, nix::FileMode, const string&, nix::Compression, nix::OpenFlags)’:
/src/src/File.cpp:29:63: error: call of overloaded ‘exists(<brace-enclosed initializer list>)’ is ambiguous
   29 |     if (mode == nix::FileMode::ReadOnly && !bfs::exists({name})) {
      |                                                               ^
In file included from /usr/include/boost/filesystem.hpp:17,
                 from /src/src/File.cpp:18:
/usr/include/boost/filesystem/operations.hpp:463:8: note: candidate: ‘bool boost::filesystem::exists(const boost::filesystem::path&)’
  463 |   bool exists(const path& p)           {return exists(detail::status(p));}
      |        ^~~~~~
In file included from /usr/include/boost/filesystem.hpp:17,
                 from /src/src/File.cpp:18:
/usr/include/boost/filesystem/operations.hpp:874:20: note: candidate: ‘bool boost::filesystem::exists(const boost::filesystem::directory_entry&)’
  874 | inline bool        exists         (const directory_entry& e) BOOST_NOEXCEPT { return filesystem::exists(e.status()); }
      |                    ^~~~~~

Closes #795

[codecov[bot]]

Codecov Report

Merging #796 into master will not change coverage. The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #796   +/-   ##
=======================================
  Coverage   95.34%   95.34%           
=======================================
  Files         197      197           
  Lines       12212    12212           
=======================================
  Hits        11644    11644           
  Misses        568      568

Impacted Files	Coverage Δ
src/File.cpp	92.85% <100%> (ø)	:arrow_up:

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[achilleas-k]

Tested locally with boost 1.71 and it works.

EDIT: Want to rebase or do an auto merge commit on GitHub?