Bump gohashtree to v0.0.3-alpha to fix ARM build issues

[mllnd]

When attempting to build the current stack on ARM-based setup, Docker build process exits with the following error message:

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 > [local-deployment_indexer build-env 7/7] RUN make -B explorer:
#63 0.571 rm -rf bin/
#63 0.576 mkdir -p bin/
#63 0.582 go run cmd/bundle/main.go
#63 22.01 # github.com/prysmaticlabs/gohashtree
#63 22.01 asm: illegal combination: 23984 (/go/pkg/mod/github.com/prysmaticlabs/gohashtree@v0.0.2-alpha/hash_arm64.s:604)	VMOV	$0, V16.S4 ZCON NONE NONE ARNG, 4 7
#63 22.01 asm: illegal combination: 23984 (/go/pkg/mod/github.com/prysmaticlabs/gohashtree@v0.0.2-alpha/hash_arm64.s:604)	VMOV	$0, V16.S4 ZCON NONE NONE ARNG, 4 7
#63 22.01 asm: illegal combination: 23984 (/go/pkg/mod/github.com/prysmaticlabs/gohashtree@v0.0.2-alpha/hash_arm64.s:604)	VMOV	$0, V16.S4 ZCON NONE NONE ARNG, 4 7
#63 22.01 asm: illegal combination: 23984 (/go/pkg/mod/github.com/prysmaticlabs/gohashtree@v0.0.2-alpha/hash_arm64.s:604)	VMOV	$0, V16.S4 ZCON NONE NONE ARNG, 4 7
#63 22.01 asm: assembly failed
#63 44.88 make: *** [Makefile:19: explorer] Error 1
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failed to solve: rpc error: code = Unknown desc = executor failed running [/bin/sh -c make -B $target]: exit code: 2

This PR bumps prysmaticlabs/gohashtree to v0.0.3-alpha, which addressed this issue with prysmaticlabs/gohashtree#8.