Improve install.sh

[YouJiacheng]

#8 (comment)

parent=$(ps -o comm $PPID |tail -1) will break docker build with error: process ID out of range if /bin/sh is actually bash or bash is explicitly used.

Reason: RUN ["/bin/bash", "-c", "/bin/bash <(curl -L micro.mamba.pm/install.sh)"] will cause the shell environment has $$=1 and $PPID=0.

As discussed in https://stackoverflow.com/questions/77524391/ppid-behavior-of-bash-c-and-dockerfile-run , https://stackoverflow.com/questions/67827986/why-is-bash-handling-child-processes-different-compared-to-sh, https://stackoverflow.com/questions/76310409/does-bash-promise-to-optimize-c-into-plain-exec-in-simple-cases, and https://unix.stackexchange.com/a/466523/537347, bash will introduce optimization that exec the last command and make $$=1 (considering the PID namespace mechanism of container).

RUN ["/bin/bash", "-c", "(/bin/bash <(curl -L micro.mamba.pm/install.sh))"] (i.e. create a subshell) or RUN ["/bin/bash", "-c", "/bin/bash <(curl -L micro.mamba.pm/install.sh); exit"] can work.

It seems that we should fallback to $$ if $PPID is not exists of is not a compatible shell.

As discussed in #8 (comment), for non-POSIX compliant shell for example fish and xonsh, install.sh might be executed by bash or sh. Thus we cannot use $$ by default.

However, using $PPID instead of $$ does introduce counter-intuitive behavior: run zsh <(curl -L micro.mamba.pm/install.sh) in a bash will init bash.
Instruction in the documentation (and README of this repo) "${SHELL}" <(curl -L https://micro.mamba.pm/install.sh) does NOT always use sh, suggesting that "${SHELL}" will be initialized (but actually NOT). And there isn't any remark/note around this instruction.

Nonetheless, allow something like INIT_WHICH_SHELL (in {bash,cmd.exe,dash,fish,posix,powershell,tcsh,xonsh,zsh}) to be set before the script is run to facilitate non-interactive installation can be really helpful.

cc @AntoinePrv

[AntoinePrv]

Thanks!