Fix: actual install command is perl-Digest-SHA

[SDawley]

Fix to previous PR. I noticed the image build failed because shasum is a perl script and not actually equivalent to sha1sum.

Issue: che-incubator/chectl#2239
The chectl package-binaries command uses shasum, so we either have to add shasum to the image used to create a workspace or add a symlink from sha256sum (which we do have).

Steps to reproduce:

• Create a Chectl workspace in the che dogfooding instance from https://github.com/SDawley/chectl/tree/contribute (my branch that contains the build fixes).
• Build Chectl
• Run the package-binaries command
• command fails with /bin/sh: shasum: command not found

Steps to verify:

• Create a Chectl workspace in the che dogfooding instance from https://github.com/SDawley/chectl/tree/contribute. It uses the latest ubi image by default.
• Build Chectl
• Run the package-binaries command

[l0rd]

@SDawley can you please update the README file too?

[SDawley]

@SDawley can you please update the README file too?

Added!
I didn't update the size since perl-Digest-SHA is 117kb installed.

[openshift-ci]

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This pull-request has been approved by: l0rd, SDawley

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