contrib: Clear HOSTVARS_MESON when cross-compiling

Signed-off-by: Steve Lhomme <robux4@ycbcr.xyz>
10 jobs for !1 with meson_native/1 in 14 seconds (queued for 9 minutes and 26 seconds)
detached
Status Job ID Name Coverage
  Build
canceled #334205
amd64 docker
android-arm

canceled #334206
amd64 docker
android-arm64

canceled #334207
amd64 docker
android-x86

canceled #334208
amd64 docker
android-x86_64

canceled #334202
amd64 docker
debian

canceled #334204
macos
ios-arm64

canceled #334203
macos
macos

canceled #334199
amd64 docker
win32

00:00:14

canceled #334200
amd64 docker
win64

canceled #334201
amd64 docker
win64-llvm