buildtest.py: Use ccache

Reduces the time to run `./buildtest.py` on my 8-core laptop from 25
minutes to 7 minutes.

```
 # Before, without ccache
$ time ./buildtest.py
[...]
./buildtest.py  5844.08s user 755.75s system 434% cpu 25:17.99 total

 # After, with ccache, empty cache.
$ time ./buildtest.py
[...]
./buildtest.py  1620.97s user 472.66s system 482% cpu 7:13.54 total
```

The `ccache -s` status after building once from an empty cache shows
just how much duplicated work is performed: 83.00% cache hits!

```
$ ccache -s
Cacheable calls:    12538 / 22769 (55.07%)
  Hits:             10407 / 12538 (83.00%)
    Direct:          9093 / 10407 (87.37%)
    Preprocessed:    1314 / 10407 (12.63%)
  Misses:            2131 / 12538 (17.00%)
Uncacheable calls:  10231 / 22769 (44.93%)
Local storage:
  Cache size (GiB):   0.1 /   5.0 ( 1.97%)
  Hits:             10407 / 12538 (83.00%)
  Misses:            2131 / 12538 (17.00%)
```

Signed-off-by: Matt Turner <mattst88@gmail.com>

Author: mattst88

.github/workflows/build-variants.yml

@@ -23,6 +23,7 @@ jobs:
       run: |
         sudo apt-get update
         sudo apt-get install -y \
+          ccache \
           docbook-xsl \
           libargon2-dev \
           libc-ares-dev \

buildtest.py

@@ -36,11 +36,15 @@
 import random
 import subprocess
 
+env = os.environ.copy()
+env['CC'] = "ccache gcc"
+env['CXX'] = "ccache g++"
+
 def run_test(msg, opts):
     subprocess.run(["make", "clean"], stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL)
     print("%s: %s" % (msg, str(opts)))
     args = ["make", "test-compile", "-j%d" % (os.cpu_count())] + opts
-    proc = subprocess.run(args, stdout=subprocess.DEVNULL)
+    proc = subprocess.run(args, stdout=subprocess.DEVNULL, env=env)
     if proc.returncode != 0:
         raise RuntimeError("BUILD FAILED: %s" % (' '.join(args)))