Use shasum if sha1sum doesn't exist in the path

[roberthbailey]

What this PR does / why we need it: bug fix for running upgrade.sh from a mac

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #37355

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This change is

[roberthbailey]

FYI, I still need to manually test this on both mac and linux, so please don't lgtm yet.

[eparis]

it does LGTM. Feel free to self label when you are confident.

[roberthbailey]

I ran the following on both a mac and linux machine:

$ git co upstream/release-1.4
$ make clean; make quick-release
$ ./cluster/kube-up.sh
$ git fetch upstream pull/37362/head:test_pull_37362
$ git co test_pull_37362
$ ./cluster/gce/upgrade.sh -M v1.5.0-beta.1

and got to the endless dots (since #37358 hasn't yet been merged) until I applied the manual fix described in #37257 (comment).

I found a minor issue on the mac side, where quoting the new variable causes bash to try and execute "shasum -a1" instead of just "shasum" passing "-a1" as an argument so I've unquoted the variable in the pipe chains and re-pushed my branch.

[roberthbailey]

Re-tested on linux after pushing the new branch and now both mac & linux appear to be working. Self-applying the lgtm label.

[k8s-github-robot]

Automatic merge from submit-queue

[jessfraz]

cherry-picked in #37864

[k8s-cherrypick-bot]

Commit found in the "release-1.4" branch appears to be this PR. Removing the "cherrypick-candidate" label. If this is an error find help to get your PR picked.