feat(arch/src): minimal-makepkg POSIX-sh build.sh generator

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <[email protected]>

testdata/golden/mytool.build.sh

@@ -0,0 +1,29 @@
+#!/bin/sh
+set -eu
+
+pkgname='mytool'
+pkgver='1.0'
+pkgrel='1'
+arch='x86_64'
+url='https://example.org'
+
+srcdir="$PWD"
+pkgdir="$DESTDIR"
+
+msg() { :; }
+
+prepare() { :; }
+
+build() {
+  cd "$srcdir"
+  ./configure --prefix=/usr
+  make
+}
+
+package() {
+  cd "$srcdir"
+  make DESTDIR="$pkgdir" install
+}
+
+build
+package