[New] `install.sh`: add `$ZDOTDIR` to zsh search #12

Job	Run time
shellcheck_matrix (bash, nvm.sh)	38s
shellcheck_matrix (sh, nvm.sh)	43s
shellcheck_matrix (dash, nvm.sh)	35s
shellcheck_matrix (ksh, nvm.sh)	38s
shellcheck_matrix (bash, install.sh)	21s
shellcheck_matrix (bash, bash_completion)	24s
shellcheck_matrix (bash, nvm-exec)	25s
shellcheck	1s
	3m 45s

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