fix pkg loading from read-only filesystems on unix

[nolta]

On our HPC systems /home is mounted read-only on the compute nodes, and if you load a precompiled pkg you'll see errors like the following:

julia> using DataFrames
ERROR: SystemError: futimes: Permission denied
Stacktrace:
  [1] systemerror(p::Symbol, errno::Int32; extrainfo::Nothing)
    @ Base ./error.jl:186
  [2] systemerror
    @ ./error.jl:185 [inlined]
  [3] touch
    @ ./filesystem.jl:361 [inlined]
  [4] touch(path::String)
    @ Base.Filesystem ./file.jl:546
  [5] _require_search_from_serialized(pkg::Base.PkgId, sourcepath::String, build_id::UInt128, stalecheck::Bool; reasons::Dict{String, Int64}, DEPOT_PATH::Vector{String})
    @ Base ./loading.jl:2090
...

But touching read-only files is supposed to be fine, according to the code comments:

julia/base/loading.jl

Lines 2058 to 2064 in 74d1230

	try
	# update timestamp of precompilation file so that it is the first to be tried by code loading
	touch(path_to_try)
	catch ex
	# file might be read-only and then we fail to update timestamp, which is fine
	ex isa IOError || rethrow()
	end

However, only IOErrors are ignored. On unix touch() calls futimes in the C stdlib and throws SystemError:

julia/base/filesystem.jl

Lines 359 to 368 in 74d1230

	function touch(f::File)
	@static if Sys.isunix()
	ret = ccall(:futimes, Cint, (Cint, Ptr{Cvoid}), fd(f), C_NULL)
	systemerror(:futimes, ret != 0)
	else
	t = time()
	futime(f, t, t)
	end
	f
	end

This patch ignores all exceptions thrown when touching precompilation files.

[StefanKarpinski]

Hi, @nolta!!!