Automatic toolchain packaging not working properly with gcc outputting relative paths

[luser]

The code we landed in #321 doesn't seem to be working exactly right. I ran a simple test distributed compile on my Linux machine and got:

luser@eye7:/build$ /build/sccache/target/debug/sccache gcc -c hello.c -o hello.o
gcc: error trying to exec 'as': execvp: No such file or directory
Compiler killed by signal 1

Looking at the contents of the toolchain tar on disk shows that as is missing:

$ tar tzf /home/luser/sccache-dist/toolchains/tc/f/f/ff6b8de934e8d03f7bf720597e3f7818bf8e8f21288c6349273e480bd335caa564b7103a7ae2655e6b1f1702a440b4d2f8d212dd11c9c0e9cfcfc7e4be209d34
tmp
etc/ld.so.conf
lib/x86_64-linux-gnu/libc.so.6
lib/x86_64-linux-gnu/libdl.so.2
lib/x86_64-linux-gnu/libm.so.6
lib/x86_64-linux-gnu/libz.so.1
lib64/ld-linux-x86-64.so.2
usr/bin/gcc
usr/lib/gcc/x86_64-linux-gnu/7/cc1
usr/lib/gcc/x86_64-linux-gnu/7/cc1plus
usr/lib/gcc/x86_64-linux-gnu/7/liblto_plugin.so
usr/lib/x86_64-linux-gnu/libgmp.so.10
usr/lib/x86_64-linux-gnu/libisl.so.19
usr/lib/x86_64-linux-gnu/libmpc.so.3
usr/lib/x86_64-linux-gnu/libmpfr.so.6

If I look at an older toolchain tar I have that was created by icecc-create-env it does include as:

$ tar tzf /home/luser/sccache-dist/toolchains/tc/6/8/6805131d38b0c2b2f8cf967c15d96e2a5ce23d26cfbc4f60f2750b5f323a4cacffb1f96149e27be05ba927b7cec674e748e0efa1e06ce2eecda4673fd1060a5d
bin/true
etc/ld.so.cache
etc/ld.so.conf
lib/x86_64-linux-gnu/libc.so.6
lib/x86_64-linux-gnu/libdl.so.2
lib/x86_64-linux-gnu/libm.so.6
lib/x86_64-linux-gnu/libz.so.1
lib64/ld-linux-x86-64.so.2
usr/bin/as
usr/bin/cc1
usr/bin/cc1plus
usr/bin/g++
usr/bin/gcc
usr/bin/objcopy
usr/lib/gcc/x86_64-linux-gnu/7/liblto_plugin.so
usr/lib/x86_64-linux-gnu/libbfd-2.30-system.so
usr/lib/x86_64-linux-gnu/libgmp.so.10
usr/lib/x86_64-linux-gnu/libisl.so.19
usr/lib/x86_64-linux-gnu/libmpc.so.3
usr/lib/x86_64-linux-gnu/libmpfr.so.6
usr/lib/x86_64-linux-gnu/libopcodes-2.30-system.so

The sccache server's log output contains:

 INFO 2019-03-04T14:19:35Z: sccache::compiler::c: Generating toolchain /usr/bin/gcc
DEBUG 2019-03-04T14:19:35Z: sccache::compiler::c: find named prog as output:
as

===

DEBUG 2019-03-04T14:19:35Z: sccache::compiler::c: find named prog objcopy output:
objcopy

===

DEBUG 2019-03-04T14:19:35Z: sccache::compiler::c: find named prog cc1 output:
/usr/lib/gcc/x86_64-linux-gnu/7/cc1

===

DEBUG 2019-03-04T14:19:35Z: sccache::compiler::c: find named prog cc1plus output:
/usr/lib/gcc/x86_64-linux-gnu/7/cc1plus

===

DEBUG 2019-03-04T14:19:35Z: sccache::compiler::c: find named file specs output:
specs

===

DEBUG 2019-03-04T14:19:35Z: sccache::compiler::c: find named file liblto_plugin.so output:
/usr/lib/gcc/x86_64-linux-gnu/7/liblto_plugin.so

===

Looking at the code involved it runs gcc -print-prog-name=as, which on my system just prints as:

$ gcc -print-prog-name=as
as

The packaging code doesn't handle non-absolute paths here, which seems to be the problem.