ci: install valgrind from snap

[tmfink]

Valgrind in Ubuntu repo shows false-positive against Rust std library.

[codecov[bot]]

Codecov Report

Merging #134 (525d790) into master (c314099) will decrease coverage by 0.62%. The diff coverage is 86.84%.

@@            Coverage Diff             @@
##           master     #134      +/-   ##
==========================================
- Coverage   94.64%   94.01%   -0.63%     
==========================================
  Files          22       22              
  Lines        2690     2691       +1     
  Branches     2651     2648       -3     
==========================================
- Hits         2546     2530      -16     
- Misses        144      161      +17     

Impacted Files	Coverage Δ
capstone-rs/src/ffi.rs	100.00% <ø> (ø)
capstone-rs/src/test.rs	96.15% <78.04%> (-1.47%)	:arrow_down:
capstone-rs/examples/demo.rs	92.30% <88.88%> (-1.93%)	:arrow_down:
capstone-rs/src/arch/arm.rs	90.27% <100.00%> (ø)
capstone-rs/src/arch/m68k.rs	92.50% <100.00%> (-0.08%)	:arrow_down:
capstone-rs/src/arch/x86.rs	100.00% <100.00%> (ø)
capstone-rs/src/capstone.rs	94.28% <100.00%> (+0.76%)	:arrow_up:
capstone-rs/src/instruction.rs	93.91% <100.00%> (ø)
capstone-rs/examples/parallel.rs	90.47% <0.00%> (-4.77%)	:arrow_down:
... and 2 more

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[tmfink]

Fixes #133