scons: use get instead of [] for ARGUMENTS

Fixes getting a not helpful error message, and gives a helpful one
instead.

Closes: mesa/mesa#2003
Fixes: 54053bc8
       ("scons: Print a deprecation warning about using scons on not windows")
51 jobs for !2477 with scons-force-scons in 30 minutes and 35 seconds (queued for 10 seconds)
latest detached
Status Job ID Name Coverage
  Container
passed debian-10:amd64 #819530

00:00:31

passed debian-10:arm64 #819532
aarch64

00:00:18

passed debian-9:amd64 #819531

00:00:22

passed lava-container:arm64 #819529

00:00:18

passed lava-container:armhf #819528

00:00:21

 
  Build
passed lava-build:arm64 #819534

00:01:38

passed lava-build:armhf #819533

00:00:40

passed meson-arm64 #819537
aarch64

00:02:48

passed meson-armhf #819536
aarch64

00:03:18

passed meson-clang #819539

00:03:07

passed meson-clover #819543

00:03:45

passed meson-clover-old-llvm #819544

00:03:35

passed meson-i386 #819546

00:02:29

passed meson-main #819535

00:05:07

passed meson-mingw32-x86_64 #819547

00:01:25

passed meson-swr-glvnd #819538

00:01:53

passed meson-vulkan #819545

00:05:34

passed meson-windows #819540
mesa-windows

00:07:09

passed scons #819548

00:07:55

passed scons-old-llvm #819549

00:05:12

passed scons-swr #819541

00:02:58

passed scons-win64 #819542

00:02:57

 
  Test
passed arm64_a306_gles2 1/4 #819574
db410c

00:03:37

passed arm64_a306_gles2 2/4 #819575
db410c

00:06:48

passed arm64_a306_gles2 3/4 #819576
db410c

00:03:37

passed arm64_a306_gles2 4/4 #819577
db410c

00:03:44

passed arm64_a630_gles2 #819563
mesa-cheza

00:04:25

passed arm64_a630_gles31 1/4 #819564
mesa-cheza

00:04:30

passed arm64_a630_gles31 2/4 #819565
mesa-cheza

00:05:07

passed arm64_a630_gles31 3/4 #819566
mesa-cheza

00:05:16

passed arm64_a630_gles31 4/4 #819567
mesa-cheza

00:05:03

passed arm64_a630_gles3 1/6 #819568
mesa-cheza

00:03:55

passed arm64_a630_gles3 2/6 #819569
mesa-cheza

00:03:20

passed arm64_a630_gles3 3/6 #819570
mesa-cheza

00:03:44

passed arm64_a630_gles3 4/6 #819571
mesa-cheza

00:04:04

passed arm64_a630_gles3 5/6 #819572
mesa-cheza

00:03:11

passed arm64_a630_gles3 6/6 #819573
mesa-cheza

00:03:41

passed lima-test:arm64 #819553
lava-meson-gxl-s905x-libretech-cc

00:04:46

passed lima-test:armhf #819552
lava-sun8i-h3-libretech-all-h3-cc

00:04:52

passed panfrost-t760-test:armhf #819550
lava-rk3288-veyron-jaq

00:05:10

passed panfrost-t860-test:arm64 #819551
lava-rk3399-gru-kevin

00:04:41

passed test-llvmpipe-gles2 1/4 #819554

00:03:13

passed test-llvmpipe-gles2 2/4 #819555

00:03:44

passed test-llvmpipe-gles2 3/4 #819556

00:03:28

passed test-llvmpipe-gles2 4/4 #819557

00:02:16

passed test-softpipe-gles2 1/4 #819558

00:01:23

passed test-softpipe-gles2 2/4 #819559

00:01:35

passed test-softpipe-gles2 3/4 #819560

00:01:29

passed test-softpipe-gles2 4/4 #819561

00:01:02

passed test-softpipe-gles3-limited #819562

00:03:57

 
  External
passed continuous-integration/appveyor/branch