change uname check in scripts/install_protoc.sh for compatibility with macos

[lugi0]

Description

The current version of the install_protoc.sh fails when running make build on MacOS due to a "bad substitution" error

./scripts/install_protoc.sh: line 6: ${OSTYPE,,}: bad substitution
make: *** [bin/protoc] Error 1

The change gets the value of $OSTYPE directly.

Furthermore, uname -s returns Darwin, which does not match against =~ darwin, causing the linux executable to be downloaded instead.
The change looks for both capitalized and non-capitalized versions of the name ([Dd]arwin).

The proposed changes have been tested both in MacOS as well as Fedora

How Has This Been Tested?

ran make build, correct version of protoc downloaded to bin folder

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• The developer has manually tested the changes and verified that the changes work.
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[tarilabs]

Furthermore, uname -s returns Darwin, which does not match against =~ darwin, causing the linux executable to be downloaded instead.

I believe the check is intended to make lowercase the return value from uname -s

See ${parameter,,pattern} from:
https://www.gnu.org/savannah-checkouts/gnu/bash/manual/bash.html#Shell-Parameter-Expansion:~:text=%24%7Bparameter%2C%2Cpattern%7D

so regardless if it's coming from uname as Darwin, darwin, dArWiN, etc it was intended to be lowercase'd and checked to be the literal darwin (use of regexp for future extension)

I believe however current PR proposal makes it less Bash-centric and more explicit, so +1

Thanks @lugi0

[tarilabs]

per comments

/lgtm
/approve

[google-oss-prow]

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