Make the package reproducible by passing the correct date

PKGBUILD

@@ -2,7 +2,7 @@
 
 pkgname=prometheus-blackbox-exporter
 pkgver=0.18.0
-pkgrel=2
+pkgrel=3
 pkgdesc="Allows blackbox probing of endpoints over HTTP, HTTPS, DNS, TCP and ICMP"
 arch=('x86_64')
 url="https://github.com/prometheus/blackbox_exporter"
@@ -30,7 +30,7 @@ build() {
       -X github.com/prometheus/common/version.Revision=$pkgver \
       -X github.com/prometheus/common/version.Branch=tarball \
       -X github.com/prometheus/common/version.BuildUser=someone@builder \
-      -X github.com/prometheus/common/version.BuildDate=$(date -d@"$SOURCE_DATE_EPOCH" +%Y%m%d-%H:%M:%S)" \
+      -X github.com/prometheus/common/version.BuildDate=$(date --date=@"$SOURCE_DATE_EPOCH" +%Y%m%d-%H:%M:%S)" \
     .
 }