In-repo-addon themes build works on Windows but not on Linux

Reproduction: https://github.com/ctusch/addon-theme-bug

When I try to build the above project on Linux I get the following error:
```
Sass Syntax Error (SassCompiler) in /tmp/broccoli-11731IvyMcUmIwccy/out-131-broccoli_merge_trees_full_application//app/styles/themes/addon-theme/app.scss:1:9

Error: Can't find stylesheet to import.
╷
1 │ @import '../../common';
│ ^^^^^^^^^^^^^^
╵
/tmp/broccoli-11731IvyMcUmIwccy/out-131-broccoli_merge_trees_full_application/app/styles/themes/addon-theme/app.scss 1:9 root stylesheet

Stack Trace and Error Report: /tmp/error.dump.f6f94bd5e855519474d4ff026a709d1e.log
```
It works on Windows. I've tried it on several different Linux machines and it always yielded the same error.

I have no idea if it's the SASS compiler, the broccoli-merge-trees plugin or ember-cli itself. It probably has something to do with the symlinks the build process uses. But I can't really check the state since the tmp directory gets cleaned up immediately.

The content of `/tmp/error.dump.f6f94bd5e855519474d4ff026a709d1e.log`:
```
=================================================================================

ENV Summary:

TIME: Wed Mar 20 2019 21:14:00 GMT+0100 (GMT+01:00)
TITLE: ember
ARGV:
- /usr/bin/node
- /home/ctusch/.config/yarn/global/node_modules/.bin/ember
- b
- --verbose
EXEC_PATH: /usr/bin/node
TMPDIR: /tmp
SHELL: /bin/bash
PATH:
- /home/ctusch/.config/yarn/global/node_modules/.bin
- /home/ctusch/bin
- /home/ctusch/.local/bin
- /usr/local/sbin
- /usr/local/bin
- /usr/sbin
- /usr/bin
- /sbin
- /bin
PLATFORM: linux x64
FREEMEM: 4779884544
TOTALMEM: 17045798912
UPTIME: 64643
LOADAVG: 0.5185546875,0.57763671875,0.5859375
CPUS:
- Intel(R) Core(TM) i7-5600U CPU @ 2.60GHz - 2594
- Intel(R) Core(TM) i7-5600U CPU @ 2.60GHz - 2594
- Intel(R) Core(TM) i7-5600U CPU @ 2.60GHz - 2594
- Intel(R) Core(TM) i7-5600U CPU @ 2.60GHz - 2594
ENDIANNESS: LE
VERSIONS:
- ares: 1.15.0
- cldr: 33.1
- http_parser: 2.8.0
- icu: 62.1
- modules: 64
- napi: 3
- nghttp2: 1.34.0
- node: 10.15.3
- openssl: 1.1.0j
- tz: 2018e
- unicode: 11.0
- uv: 1.23.2
- v8: 6.8.275.32-node.51
- zlib: 1.2.11

ERROR Summary:

- broccoliBuilderErrorStack: Error: Error: Can't find stylesheet to import.
╷
1 │ @import '../../common';
│ ^^^^^^^^^^^^^^
╵
/tmp/broccoli-11731IvyMcUmIwccy/out-131-broccoli_merge_trees_full_application/app/styles/themes/addon-theme/app.scss 1:9 root stylesheet
at Object.BH (/mnt/c/tmp/addon-theme-bug/node_modules/sass/sass.dart.js:5229:30)
at Object.Eh (/mnt/c/tmp/addon-theme-bug/node_modules/sass/sass.dart.js:5145:34)
at wO.$2 (/mnt/c/tmp/addon-theme-bug/node_modules/sass/sass.dart.js:15295:20)
at uP.vq (/mnt/c/tmp/addon-theme-bug/node_modules/sass/sass.dart.js:9034:42)
at uP.vp (/mnt/c/tmp/addon-theme-bug/node_modules/sass/sass.dart.js:9036:32)
at iy.uC (/mnt/c/tmp/addon-theme-bug/node_modules/sass/sass.dart.js:8384:46)
at uo.$0 (/mnt/c/tmp/addon-theme-bug/node_modules/sass/sass.dart.js:8526:7)
at Object.eG (/mnt/c/tmp/addon-theme-bug/node_modules/sass/sass.dart.js:1512:80)
at ad.ba (/mnt/c/tmp/addon-theme-bug/node_modules/sass/sass.dart.js:8447:3)
at iM.ba (/mnt/c/tmp/addon-theme-bug/node_modules/sass/sass.dart.js:8377:25)
- code: [undefined]
- codeFrame: Error: Can't find stylesheet to import.
╷
1 │ @import '../../common';
│ ^^^^^^^^^^^^^^
╵
/tmp/broccoli-11731IvyMcUmIwccy/out-131-broccoli_merge_trees_full_application/app/styles/themes/addon-theme/app.scss 1:9 root stylesheet
- errorMessage: /tmp/broccoli-11731IvyMcUmIwccy/out-131-broccoli_merge_trees_full_application//app/styles/themes/addon-theme/app.scss:1:10: Error: Can't find stylesheet to import.
╷
1 │ @import '../../common';
│ ^^^^^^^^^^^^^^
╵
/tmp/broccoli-11731IvyMcUmIwccy/out-131-broccoli_merge_trees_full_application/app/styles/themes/addon-theme/app.scss 1:9 root stylesheet
at SassCompiler
- errorType: Sass Syntax Error
- location:
- column: 9
- file: /tmp/broccoli-11731IvyMcUmIwccy/out-131-broccoli_merge_trees_full_application//app/styles/themes/addon-theme/app.scss
- line: 1
- treeDir: [undefined]
- message: /tmp/broccoli-11731IvyMcUmIwccy/out-131-broccoli_merge_trees_full_application//app/styles/themes/addon-theme/app.scss:1:10: Error: Can't find stylesheet to import.
╷
1 │ @import '../../common';
│ ^^^^^^^^^^^^^^
╵
/tmp/broccoli-11731IvyMcUmIwccy/out-131-broccoli_merge_trees_full_application/app/styles/themes/addon-theme/app.scss 1:9 root stylesheet
at SassCompiler
- name: BuildError
- nodeAnnotation: [undefined]
- nodeName: SassCompiler
- originalErrorMessage: Error: Can't find stylesheet to import.
╷
1 │ @import '../../common';
│ ^^^^^^^^^^^^^^
╵
/tmp/broccoli-11731IvyMcUmIwccy/out-131-broccoli_merge_trees_full_application/app/styles/themes/addon-theme/app.scss 1:9 root stylesheet
- stack: Error: Error: Can't find stylesheet to import.
╷
1 │ @import '../../common';
│ ^^^^^^^^^^^^^^
╵
/tmp/broccoli-11731IvyMcUmIwccy/out-131-broccoli_merge_trees_full_application/app/styles/themes/addon-theme/app.scss 1:9 root stylesheet
at Object.BH (/mnt/c/tmp/addon-theme-bug/node_modules/sass/sass.dart.js:5229:30)
at Object.Eh (/mnt/c/tmp/addon-theme-bug/node_modules/sass/sass.dart.js:5145:34)
at wO.$2 (/mnt/c/tmp/addon-theme-bug/node_modules/sass/sass.dart.js:15295:20)
at uP.vq (/mnt/c/tmp/addon-theme-bug/node_modules/sass/sass.dart.js:9034:42)
at uP.vp (/mnt/c/tmp/addon-theme-bug/node_modules/sass/sass.dart.js:9036:32)
at iy.uC (/mnt/c/tmp/addon-theme-bug/node_modules/sass/sass.dart.js:8384:46)
at uo.$0 (/mnt/c/tmp/addon-theme-bug/node_modules/sass/sass.dart.js:8526:7)
at Object.eG (/mnt/c/tmp/addon-theme-bug/node_modules/sass/sass.dart.js:1512:80)
at ad.ba (/mnt/c/tmp/addon-theme-bug/node_modules/sass/sass.dart.js:8447:3)
at iM.ba (/mnt/c/tmp/addon-theme-bug/node_modules/sass/sass.dart.js:8377:25)

=================================================================================
```

---

Output from `ember version --verbose && npm --version && yarn --version`:
```
ember-cli: 3.8.1
http_parser: 2.8.0
node: 10.15.3
v8: 6.8.275.32-node.51
uv: 1.23.2
zlib: 1.2.11
ares: 1.15.0
modules: 64
nghttp2: 1.34.0
napi: 3
openssl: 1.1.0j
icu: 62.1
unicode: 11.0
cldr: 33.1
tz: 2018e
os: linux x64
6.4.1
1.15.2
```