travis: don't download static apk if available on the system

Just download the APK tools if the install-alpine script is running in
a system that doesn't have APK installed. This is needed for upcoming
ppc64le builder.

.travis/install-alpine

@@ -7,15 +7,18 @@ set -eu
 APK_TOOLS_URI='https://github.com/alpinelinux/apk-tools/releases/download/v2.7.2/apk-tools-2.7.2-x86_64-linux.tar.gz'
 APK_TOOLS_SHA256='e949d3826d08ad41622b29186f960b135711dd96aacfb80ff8238c0fd69d051f'
 
+APK=$(which apk 2>/dev/null) || true
 
-title 'Downloading static apk-tools'
-
-cd /tmp
-wget -T 10 "$APK_TOOLS_URI"
-echo "$APK_TOOLS_SHA256  ${APK_TOOLS_URI##*/}" | sha256sum -c
-tar -xzf ${APK_TOOLS_URI##*/}
-mv apk-tools-*/apk apk
+if [ ! -e "$APK" ]; then
+	title 'Downloading static apk-tools'
 
+	cd /tmp
+	wget -T 10 "$APK_TOOLS_URI"
+	echo "$APK_TOOLS_SHA256  ${APK_TOOLS_URI##*/}" | sha256sum -c
+	tar -xzf ${APK_TOOLS_URI##*/}
+	mv apk-tools-*/apk /tmp/apk
+	APK=/tmp/apk
+fi
 
 title 'Installing Alpine Linux'
 
@@ -26,7 +29,7 @@ echo "$MIRROR_URI/main" > etc/apk/repositories
 cp -R "$CLONE_DIR"/.travis/keys etc/apk/keys
 cp /etc/resolv.conf etc/resolv.conf
 
-/tmp/apk \
+$APK \
 	--root . --update-cache --initdb --no-progress \
 	add alpine-base