[Fix] `nvm_get_arch`: proper value for alpine linux #1123

Job	Run time
shellcheck_matrix (bash, nvm.sh)	41s
shellcheck_matrix (sh, nvm.sh)	45s
shellcheck_matrix (dash, nvm.sh)	41s
shellcheck_matrix (ksh, nvm.sh)	1m 2s
shellcheck_matrix (bash, install.sh)	17s
shellcheck_matrix (bash, bash_completion)	31s
shellcheck_matrix (bash, nvm-exec)	27s
shellcheck	9s
	4m 33s

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