fix isvalid_cache_header logic

[IanButterworth]

Before this I was getting

julia> info_cachefile("Revise")
ERROR: TypeError: non-boolean (UInt64) used in boolean context
Stacktrace:
 [1] macro expansion
   @ ~/Documents/GitHub/PkgCacheInspector.jl/src/PkgCacheInspector.jl:107 [inlined]
 [2] macro expansion
   @ ./lock.jl:267 [inlined]
 [3] info_cachefile(pkg::Base.PkgId, path::String)
   @ PkgCacheInspector ~/Documents/GitHub/PkgCacheInspector.jl/src/PkgCacheInspector.jl:103
 [4] info_cachefile(pkg::Base.PkgId)
   @ PkgCacheInspector ~/Documents/GitHub/PkgCacheInspector.jl/src/PkgCacheInspector.jl:136
 [5] info_cachefile(pkgname::String)
   @ PkgCacheInspector ~/Documents/GitHub/PkgCacheInspector.jl/src/PkgCacheInspector.jl:139
 [6] top-level scope
   @ REPL[2]:1

With this

julia> info_cachefile("Revise")
Contents of /Users/ian/.julia/compiled/v1.10/Revise/M1Qoh_1DCqx.ji:
  modules: Any[Revise]
  init order: Any[Revise]
  115 external methods
  1701 new specializations of external methods (Base 81.5%, Base.Iterators 3.9%, Base.Broadcast 3.1%, ...)
  347 external methods with new roots
  12497 external targets
  9370 edges
  Segment sizes (bytes):
  system:      3304304 ( 55.69%)
  isbits:      2304751 ( 38.84%)
  symbols:       26597 (  0.45%)
  tags:          45520 (  0.77%)
  relocations:  252605 (  4.26%)
  gvars:             0 (  0.00%)
  fptrs:             0 (  0.00%)