Build details: use first command to infer build length

[humitos]

We have this code

ext-theme/src/js/build/detail.js

Lines 320 to 338 in 7115c24

	this.length.subscribe((length) => {
	let duration;
	if (length) {
	duration = dayjs.duration(length, "seconds");
	} else {
	// Infer length from build start time
	const dateNow = dayjs();
	const dateStart = dayjs(this.date());
	duration = dayjs.duration(dateNow.diff(dateStart));
	}
	let formatParts = ["s[s]"];
	if (duration.minutes()) {
	formatParts.unshift("m[m]");
	}
	if (duration.hours()) {
	formatParts.unshift("H[h]");
	}
	this.length_display(duration.format(formatParts.join(" ")));
	});

that infers the build duration based on Build.date. However, that date may not be accurate if the build was not immediate executed, or if it was retried a few times.

To make this infer more accurate, we should use the BuildCommand.start_date from the first build command (usually, git clone). That will give use a more realistic value here.