minimize impact of invalidations due to uninferred `ctx`

[thchr]

I originally tried to fix this in SHA.jl, but it is more probably more appropriate to fix it here. The problem is summarized here: https://github.com/JuliaCrypto/SHA.jl/pull/88#issuecomment-1516151954

[codecov[bot]]

Codecov Report

Patch coverage: 85.71% and project coverage change: -0.12 :warning:

Comparison is base (b0b8ec7) 97.38% compared to head (ae8e138) 97.27%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #158 +/- ## ========================================== - Coverage 97.38% 97.27% -0.12% ========================================== Files 4 4 Lines 803 807 +4 ========================================== + Hits 782 785 +3 - Misses 21 22 +1 ``` | [Impacted Files](https://codecov.io/gh/JuliaIO/Tar.jl/pull/158?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaIO) | Coverage Δ | | |---|---|---| | [src/extract.jl](https://codecov.io/gh/JuliaIO/Tar.jl/pull/158?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaIO#diff-c3JjL2V4dHJhY3Quamw=) | `98.15% <ø> (ø)` | | | [src/Tar.jl](https://codecov.io/gh/JuliaIO/Tar.jl/pull/158?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaIO#diff-c3JjL1Rhci5qbA==) | `91.34% <85.71%> (-0.66%)` | :arrow_down: |

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[thchr]

Gentle bump :)