Avoid breaking the cat pipe by reading from head (infra)

[Hook25]

Description

Currenty the frontend snaps are failing to build. In the logs snapcraft is reporting a non-0 return value on the new workaround part. This error is 141 (broken pipe). This happens because we are cat-ing from /dev/urandom (an infinite source) and closing the pipe via head, effectively breaking it. This doesn't make some series fail because it may be (not confirmed, but a very likely theory) that snapcraft started to set the set -o pipefail only recently-ish.

This PR fixes this problem by making head read the file directly instead of reading it via cat

Resolved issues

Broken daily builds here: https://github.com/canonical/checkbox/actions/runs/7505763759

Documentation

N/A

This doesn't change the effect of the command, it should only make it go through without "breaking" the pipe and making snapcraft mad

Tests

The following fails (exits with return code 141) with the same error:

!/bin/bash

set -o pipefail
set -e

cat /dev/urandom | head -c 1000 > /dev/null

while this doesn't:

!/bin/bash

set -o pipefail
set -e

head -c 1000 /dev/urandom > /dev/null

[yphus]

Tested with the checkbox classic20 frontend snap:

Building workaround-automated-review-issue 
+ echo 'This file is a workaround for a bug in the automated snap review tool'
+ echo 'this only contains random bytes to pad the snap to 16kb'
+ echo 'see: https://bugs.launchpad.net/review-tools/+bug/2049093'
+ head -c 16384 /dev/urandom
Staging config-variables 
+ snapcraftctl stage
Staging launchers 
+ snapcraftctl stage
Staging version-calculator 
+ snapcraftctl stage
Staging workaround-automated-review-issue 
+ snapcraftctl stage
Priming config-variables 
+ snapcraftctl prime
Priming launchers 
+ snapcraftctl prime
Priming version-calculator 
+ snapcraftctl prime
Priming workaround-automated-review-issue 
+ snapcraftctl prime
The 'passthrough' property is being used to propagate experimental properties to snap.yaml that have not been validated.
Snapping |                                                                                        
Snapped checkbox_3.3.0-dev87_amd64.snap