Use thread-safe call to ffmpeg in Julia v1.6

[giordano]

Interestingly enough, this makes exe a bit more type-stable (a union type
instead of Any):

before:

julia> @code_warntype FFMPEG.exe(`-version`);
Variables
  #self#::Core.Const(FFMPEG.exe)
  arg::Cmd

Body::Any
1 ─ %1 = FFMPEG.:(var"#exe#5")(FFMPEG.ffmpeg, false, #self#, arg)::Any
└──      return %1

after:

julia> @code_warntype FFMPEG.exe(`-version`);
Variables
  #self#::Core.Const(FFMPEG.exe)
  arg::Cmd

Body::Union{Base.Process, Vector{String}}
1 ─ %1 = FFMPEG.:(var"#exe#10")(FFMPEG.ffmpeg, false, #self#, arg)::Union{Base.Process, Vector{String}}
└──      return %1

[codecov]

Codecov Report

Merging #47 (cac1f04) into master (a819cde) will decrease coverage by 2.32%.
The diff coverage is 66.66%.

❗ Current head cac1f04 differs from pull request most recent head bb4eee3. Consider uploading reports for the commit bb4eee3 to get more accurate results

@@             Coverage Diff             @@
##            master      #47      +/-   ##
===========================================
- Coverage   100.00%   97.67%   -2.33%     
===========================================
  Files            1        1              
  Lines           44       43       -1     
===========================================
- Hits            44       42       -2     
- Misses           0        1       +1     

Impacted Files	Coverage Δ
src/FFMPEG.jl	97.67% <66.66%> (-2.33%)	⬇️

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