Made the build step timeout a template input (#791)

The build step is capped at fifteen minutes for every consumer, on the same
shape of assumption the install step carried: that a consumer builds once.
Measured build steps are 0.5 minutes for LevelX, 1.1 for FileX and 6.4 for
USBX, so fifteen is right for them.

GUIX builds five configurations in that step, because its suite is staged
across the three steps, and reaches 11.8 minutes. That is 79 per cent of the
cap, and it grows with every regression test added -- six advisories landed
tests this week.

The cap is now an input defaulting to fifteen, matching install_timeout_minutes
above. No consumer changes unless it asks to.

Assisted-by: Claude Code (Opus 5) <noreply@anthropic.com>

.github/workflows/regression_template.yml

@@ -32,6 +32,15 @@ on:
         default: 10
         required: false
         type: number
+      # Minutes for the build step, on the same reasoning as the install step
+      # above. Fifteen suits a consumer that builds once: measured build steps
+      # are 0.5 minutes for LevelX, 1.1 for FileX and 6.4 for USBX. GUIX builds
+      # five configurations here as part of its staging and reaches 11.8, which
+      # is 79 per cent of the cap and shrinking as tests are added.
+      build_timeout_minutes:
+        default: 15
+        required: false
+        type: number
       skip_test:
         default: false
         required: false
@@ -127,7 +136,7 @@ jobs:
     # test: the build is where -fprofile-arcs is decided, and the test run is
     # where the reports are collected and merged.
     - name: Build
-      timeout-minutes: 15
+      timeout-minutes: ${{ inputs.build_timeout_minutes }}
       env:
         TX_COVERAGE: ${{ inputs.skip_coverage && 'OFF' || 'ON' }}
       run: ${{ inputs.build_script }}