[Fix] `install.sh`: when `NVM_INSTALL_GITHUB_REPO` is set, issue a wa… #1129

Job	Run time
shellcheck_matrix (bash, nvm.sh)	44s
shellcheck_matrix (sh, nvm.sh)	50s
shellcheck_matrix (dash, nvm.sh)	51s
shellcheck_matrix (ksh, nvm.sh)	48s
shellcheck_matrix (bash, install.sh)	29s
shellcheck_matrix (bash, bash_completion)	27s
shellcheck_matrix (bash, nvm-exec)	23s
shellcheck	8s
	4m 40s

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