Tests fail if /etc/alpine-release exists

Hello, I noticed this test failure:

```
--- FAIL: TestParseApkInstalled_Shuffled (0.00s)
apk-installed_test.go:92: Did not expect apk-tools@2.12.10-r1 (Alpine:v3.21, 0188f510baadbae393472103427b9c1875117136, )
apk-installed_test.go:92: Did not find apk-tools@2.12.10-r1 (Alpine, 0188f510baadbae393472103427b9c1875117136, )
--- FAIL: TestParseApkInstalled_Single (0.00s)
apk-installed_test.go:72: Did not expect apk-tools@2.12.10-r1 (Alpine:v3.21, 0188f510baadbae393472103427b9c1875117136, )
apk-installed_test.go:72: Did not find apk-tools@2.12.10-r1 (Alpine, 0188f510baadbae393472103427b9c1875117136, )
--- FAIL: TestParseApkInstalled_Multiple (0.00s)
apk-installed_test.go:112: Did not expect alpine-baselayout-data@3.4.0-r0 (Alpine:v3.21, bd965a7ebf7fd8f07d7a0cc0d7375bf3e4eb9b24, )
apk-installed_test.go:112: Did not expect musl@1.2.3-r4 (Alpine:v3.21, f93af038c3de7146121c2ea8124ba5ce29b4b058, )
apk-installed_test.go:112: Did not expect busybox@1.35.0-r29 (Alpine:v3.21, 1dbf7a793afae640ea643a055b6dd4f430ac116b, )
apk-installed_test.go:112: Did not find alpine-baselayout-data@3.4.0-r0 (Alpine, bd965a7ebf7fd8f07d7a0cc0d7375bf3e4eb9b24, )
apk-installed_test.go:112: Did not find musl@1.2.3-r4 (Alpine, f93af038c3de7146121c2ea8124ba5ce29b4b058, )
apk-installed_test.go:112: Did not find busybox@1.35.0-r29 (Alpine, 1dbf7a793afae640ea643a055b6dd4f430ac116b, )
--- FAIL: TestParseApkInstalled_Malformed (0.00s)
apk-installed_test.go:52: Did not expect busybox@ (Alpine:v3.21, 1dbf7a793afae640ea643a055b6dd4f430ac116b, )
apk-installed_test.go:52: Did not find busybox@ (Alpine, 1dbf7a793afae640ea643a055b6dd4f430ac116b, )
FAIL
FAIL github.com/google/osv-scanner/pkg/lockfile 0.095s
```

I checked the code and it seems `pkg/lockfile/apk-installed.go` tests for `/etc/alpine-release`, which is present when building this on the Alpine build servers.

I'm not sure what's the best way to fix this however.